Most modern studies of Saint Patrick follow a variant of T. F. O'Rahilly's "Two Patricks" theory. That is to say, many of the traditions later attached to Saint Patrick originally concerned Palladius, who Prosper of Aquitaine's Chronicle says was sent by Pope Celestine I as the first bishop to Irish Christians in 431. Palladius was not the only early cleric in Ireland at this time. Saints Auxilius, Secundinus and Iserninus are associated with early churches in Munster and Leinster. By this reading, Palladius was active in Ireland until the 460s.

Prosper associates Palladius' appointment with the visits of Germanus of Auxerre to Britain to suppress the Pelagian heresy and it has been suggested that Palladius and his colleagues were sent to Ireland to ensure that exiled Pelagians did not establish themselves among the Irish Christians. The appointment of Palladius and his fellow-bishops was not obviously a mission to convert the Irish, but more probably intended to minister to existing Christian communities in Ireland. The sites of churches associated with Palladius and his colleagues are close to royal centres of the period: Secundus is remembered by Dunshaughlin, County Meath, close to the Hill of Tara which is associated with the High King of Ireland; Killashee, County Kildare, close to Naas with links with the Kings of Leinster, is probably named for Auxilius. This activity was limited to the southern half of Ireland, and there is no evidence for them in Ulster or Connacht.

Although the evidence for contacts with Gaul is clear, the borrowings from Latin into the Old Irish language show that links with former Roman Britain were many. Saint Iserninus, who appears to be of the generation of Palladius, is thought to have been a Briton, and is associated with the lands of the Uí Cheinnselaig in Leinster. The Palladian mission should not be contrasted with later "British" missions, but forms a part of them.

Two Latin letters survive which are generally accepted to have been written by Patrick. These are the Declaration () and the Letter to the soldiers of Coroticus (). The Declaration is the more important of the two. In it Patrick gives a short account of his life and his mission.

Patrick was born in Roman Britain at Banna Venta Berniae, a location otherwise unknown. Calpornius, his father, was a deacon, his grandfather Potitus a priest. When he was about sixteen, he was captured and carried off as a slave to Ireland. Patrick worked as a herdsman, remaining a captive for six years. He writes that his faith grew in captivity, and that he prayed daily. After six years he heard a voice telling him that he would soon go home, and then that his ship was ready. Fleeing his master, he travelled to a port, two hundred miles away he says, where he found a ship and, after various adventures, returned home to his family, now in his early twenties.

Patrick recounts that he had a vision a few years after returning home:

I saw a man coming, as it were from Ireland. His name was Victoricus, and he carried many letters, and he gave me one of them. I read the heading: "The Voice of the Irish". As I began the letter, I imagined in that moment that I heard the voice of those very people who were near the wood of Foclut, which is beside the western sea—and they cried out, as with one voice: "We appeal to you, holy servant boy, to come and walk among us.

A. B. E. Hood suggests that the Victoricus of Patrick's vision may be identified with Saint Victricius, bishop of Rouen in the late 4th century, who was the only European churchman of the time to advocate or practice conversion of pagans, and who visited Britain in an official capacity in 396.

Much of the Declaration concerns charges made against Patrick by his fellow Christians at a trial. What these charges were, he does not say explicitly, but he writes that he returned the gifts which wealthy women gave him, did not accept payment for baptisms, nor for ordaining priests, and indeed paid for many gifts to kings and judges, and paid for the sons of chiefs to accompany him. It is concluded, therefore, that he was accused of some sort of financial impropriety, and perhaps of having obtained his bishopric in Ireland with personal gain in mind.

From this same evidence, something can be seen of Patrick's mission. He writes that he "baptised thousands of people". He ordained priests to lead the new Christian communities. He converted wealthy women, some of whom became nuns in the face of family opposition. He also dealt with the sons of kings, converting them too.

Patrick's position as a foreigner in Ireland was not an easy one. His refusal to accept gifts from kings placed him outside the normal ties of kinship, fosterage and affinity. Legally he was without protection, and he says that he was on one occasion beaten, robbed of all he had, and put in chains, perhaps awaiting execution.

Murchiú's life of Saint Patrick contains a supposed prophecy by the druids which gives an impression of how Patrick and other Christian missionaries were seen by those hostile to them:

Across the sea will come Adze-head, crazed in the head, his cloak with hole for the head, his stick bent in the head.He will chant impieties from a table in the front of his house; all his people will answer: "so be it, so be it."

The second piece of evidence that comes from Patrick's life is the Letter to Coroticus or Letter to the Soldiers of Coroticus, written after a first remonstrance was received with ridicule and insult. In this, Patrick writes an open letter announcing that he has excommunicated Coroticus because he had taken some of Patrick's converts into slavery while raiding in Ireland. The letter describes the followers of Coroticus as "fellow citizens of the devils" and "associates of the Scots [ie, the Irish of Argyll and northern Ireland] and Apostate Picts". Based largely on an 8th-century gloss, Coroticus is taken to be King Ceretic of Alt Clut. It has been suggested that it was the sending of this letter which provoked the trial which Patrick mentions in the Confession.

According to the latest reconstruction of the old Irish annals, Patrick died in AD 461 on March 17, a date accepted by some modern historians. Prior to the 1940s it was believed without doubt that he died in 420 and thus had lived in the first half of the 5th century. A lecture entitled "The Two Patricks", published in 1942 by T. F. O'Rahilly, caused enormous controversy by proposing that there had been two "Patricks", Palladius and Patrick, and that what we now know of St. Patrick was in fact in part a conscious effort to blend the two into one hagiographic personality. Decades of contention eventually ended with most historians now asserting that Patrick was indeed most likely to have been active in the mid-to-late 5th century.

While Patrick's own writings contain no dates, they do contain information which can be used to date them. Patrick's quotations from the Acts of the Apostles follow the Vulgate, strongly suggesting that his ecclesiastical conversion did not take place before the early 5th century. Patrick also refers to the Franks as being pagans. Their conversion is dated to the period 496–508.

There is plentiful evidence for a medieval tradition that Patrick had died in 493. An addition to the Annals of Ulster states that in the year 553 (approximately two hundred and fifty years before the addition was made):

I have found this in the Book of Cuanu: The relics of Patrick were placed sixty years after his death in a shrine by Colum Cille. Three splendid halidoms were found in the burial-place: his goblet, the Angel's Gospel, and the Bell of the Testament. This is how the angel distributed the halidoms: the goblet to Dún, the Bell of the Testament to Ard Macha, and the Angel's Gospel to Colum Cille himself. The reason it is called the Angel's Gospel is that Colum Cille received it from the hand of the angel.

The placing of this event in the year 553 indicate a tradition that Patrick's death was 493, or at least in the early years of that decade, and the Annals of Ulster report under 493:

Patrick, arch-apostle, or archbishop and apostle of the Irish, rested on the 16th of the Kalends of April in the 120th year of his age, in the 60th year after he had come to Ireland to baptise the Irish.

This tradition is also seen in an annalistic reference to the death of a saint termed Patrick's disciple, Mochta, who is said to have died in 535.

St. Patrick is said to be buried at Down Cathedral in Downpatrick, County Down, alongside St. Brigid and St. Columba, although this has never been proven. The Battle for the Body of St. Patrick demonstrates the importance of both him as a spiritual leader, and of his body as an object of veneration, in early Christian Ireland. Saint Patrick Visitor Centre is a modern exhibition complex located in Downpatrick and is a permanent interpretative exhibition centre featuring interactive displays on the life and story of Saint Patrick. It provides the only permanent exhibition centre in the world devoted to Saint Patrick.

An early document which is silent concerning Patrick is the letter of Columbanus to Pope Boniface IV of about 613. Columbanus writes that Ireland's Christianity "was first handed to us by you, the successors of the holy apostles", apparently referring to Palladius only, and ignoring Patrick. Writing on the Easter controversy in 632 or 633, Cummian...it is uncertain whether this is the Cummian associated with Clonfert or Cumméne of Iona...does refer to Patrick, calling him our papa, that is pope or primate.

Two works by late seventh-century hagiographers of Patrick have survived. These are the writings of Tírechán, and Vita sancti Patricii of Muirchu moccu Machtheni. Both writers relied upon an earlier work, now lost, the Book of Ultán. This Ultán, probably the same person as Ultan of Ardbraccan, was Tírechán's foster-father. His obituary is given in the Annals of Ulster under the year 657. These works thus date from a century and a half after Patrick's death.

Tírechán writes

"I found four names for Patrick written in the book of Ultán, bishop of the tribe of Conchobar: holy Magonus (that is, "famous"); Succetus (that is, the god of war); Patricius (that is, father of the citizens); Cothirtiacus (because he served four houses of druids)."

Muirchu records much the same information, adding that "[h]is mother was named Concessa." The name Cothirtiacus, however, is simply the latinized form of Old Irish Cothraige, which is the Q-Celtic form of Latin Patricius.

The Patrick portrayed by Tírechán and Muirchu is a martial figure, who contests with druids, overthrows pagan idols, and curses kings and kingdoms. On occasion, their accounts contradict Patrick's own writings: Tírechán states that Patrick accepted gifts from female converts although Patrick himself flatly denies this. However, the emphasis Tírechán and Muirchu placed on female converts, and in particular royal and noble women who became nuns, is thought to be a genuine insight into Patrick's work of conversion. Patrick also worked with the unfree and the poor, encouraging them to vows of monastic chastity. Tírechán's account suggests that many early Patrician churches were combined with nunneries founded by Patrick's noble female converts.

The martial Patrick found in Tírechán and Muirchu, and in later accounts, echoes similar figures found during the conversion of the Roman Empire to Christianity. It may be doubted whether such accounts are an accurate representation of Patrick's time, although such violent events may well have occurred as Christians gained in strength and numbers.

Much of the detail supplied by Tírechán and Muirchu, in particular the churches established by Patrick, and the monasteries founded by his converts, may relate to the situation in the 7th century, when the churches which claimed ties to Patrick, and in particular Armagh, were expanding their influence throughout Ireland in competition with the church of Kildare. In the same period, Wilfred, Archbishop of York, claimed to speak, as metropolitan archbishop, "for all the northern part of Britain and of Ireland" at a council held in Rome in the time of Pope Agatho, thus claiming jurisdiction over the Irish church.

Other presumed early materials include the Irish annals, which contain records from the Chronicle of Ireland. These sources have conflated Palladius and Patrick. Another early document is the so-called First Synod of Saint Patrick. This is a seventh-century document, once, but no longer, taken as to contain a 5th century original text. It apparently collects the results of several early synods, and represents an era when pagans were still a major force in Ireland. The introduction attributes it to Patrick, Auxilius, and Iserninus, a claim which "cannot be taken at face value."

Pious legend credits Patrick with banishing snakes from the island, though all evidence suggests that post-glacial Ireland never had snakes; one suggestion is that snakes referred to the serpent symbolism of the Druids of that time and place, as shown for instance on coins minted in Gaul (see Carnutes), or that it could have referred to beliefs such as Pelagianism, symbolised as “serpents”. Legend also credits Patrick with teaching the Irish about the concept of the Trinity by showing people the shamrock, a 3-leaved clover, using it to highlight the Christian belief of 'three divine persons in the one God' (as opposed to the Arian belief that was popular in Patrick's time).

Some Irish legends involve the Oilliphéist, the Caoránach, and the Copóg Phádraig. During his evangelising journey back to Ireland from his parent's home at Birdoswald, he is understood to have carried with him an ash wood walking stick or staff. He thrust this stick into the ground wherever he was evangelising and at the place now known as Aspatria (ash of Patrick) the message of the dogma took so long to get through to the people there that the stick had taken root by the time he was ready to move on.

The 12th century work Acallam na Senórach tells of Patrick being met by two ancient warriors, Caílte mac Rónáin and Oisín, during his evangelical travels. The two were once members of Fionn mac Cumhaill's warrior band the Fianna, and somehow survived to Patrick's time. They traveled with the saint and told him their stories.

The National Museum of Ireland, in Dublin possesses a bell first mentioned, according to the Annals of Ulster, in the Book of Cuanu in the year 552. The bell was part of a collection of "relics of Patrick" robbed from his tomb sixty years after his death by Colum Cille to be placed in a shrine. The bell is described as "The Bell of the Testament", one of three relics of "precious minna" (extremely valuable items), of which the other two are described as Patrick's goblet and "The Angels Gospel". Cille is described to have been under the direction of an "Angel" for whom he sent the goblet to Down, the bell to Armagh, and kept possession of the Angels Gospel for himself. The name Angels Gospel is given to the book because it was supposed that Cille received it from the angel's hand. A stir was caused in 1044 when two kings, in some dispute over the bell, went on spates of prisoner taking and cattle theft. The annals make one more apparent reference to the bell when chronicling a death, of 1356, "Solomon Ua Mellain, The Keeper of The Bell of the Testament, protector, rested in Christ." As a museum exhibit, the bell is accompanied by a shrine in which it was encased for King Domnall Ua Lochlainn sometime between 1091 and 1105. The shrine is a sparkling example of fine jewellry. Intricate and delicate Celtic design has been worked in gold and silver over every surface except where encrusted with large precious stones. The Bell was inscribed in Gaelic: "U INMAINEN" (which translates to: NOONAN) "who with his sons enriched/decorated it" (metal work was often inscribed for remembrance).

Although today one or two of the jewels are missing as well as some of the panels of Celtic artwork, full appreciation of the shrine's workmanship is unaffected and it is kept, along with Patrick's Bell, in glittering condition by the National Museum as a priceless national treasure. The bell itself is simple in design, hammered into shape with a small handle fixed to the top with rivets. Originally forged from iron, it has since been coated in bronze. The shrine is inscribed with three names, including King Domnall Ua Lochlainn's. The rear of the shrine, not intended to be seen, is decorated with crosses while the handle is decorated with, among other work, Celtic designs of birds. The bell is accredited with working a miracle in 1044 and having been coated in bronze to shield it from human eyes, for which it would be too holy. It measures 12.5 x 10 cm at the base, 12.8 x 4 cm at the shoulder, 16.5 cm from base to shoulder, 3.3 cm from shoulder to top of handle and weighs 1.7 kg.

Patrick's Bell and shrine were featured on RTE's the Late Late Show, the world's longest-running talk show, in March 2008 along with part of the 2000-year-old Broighter Hoard to mark celebrations for St Patrick's Day.

March 17, popularly known as St. Patrick's Day, is believed to be his death date and is the date celebrated as his feast day. The day became a feast day in the universal church due to the influence of the Waterford-born Franciscan scholar Luke Wadding, as a member of the commission for the reform of the Breviary in the early part of the 17th century.

For most of Christianity's first thousand years, canonisations were done on the diocesan or regional level. Relatively soon after the death of people considered to be very holy people, the local Church affirmed that they could be liturgically celebrated as saints. As a result, St. Patrick has never been formally canonised by a Pope; nevertheless, various Christian churches declare that he is a Saint in Heaven (he is in the List of Saints). He is still widely venerated in Ireland and elsewhere today.

St. Patrick is honored with a feast day on the liturgical calendar of the Episcopal Church on March 17.

St. Patrick is also venerated in the Orthodox Church, especially among English-speaking Orthodox Christians living in the United Kingdom and the Republic of Ireland and in North America. There are Orthodox icons dedicated to him.

Robert Southey wrote a ballad called Saint Patrick's Purgatory, based on popular legends surrounding the saint's name.Stephen R. Lawhead also wrote the fictional Patrick: Son of Ireland based on the life of the celebrated Saint.

Dutch/Scottish singer Chris Anderson wrote a poem called Saint Patrick's Lament, based on the Saint's remembrance festival Saint Patrick's Day.