Born in Belfast in 1929, he grew up in Dublin, attending O'Connell School and Belvedere College. First place in Ireland in French and German in the Leaving Certificate was followed by a scholarship in Classical Languages for University College Dublin, which he entered in 1947. While completing a BA there in History and Economics, he studied English and Spanish in Trinity College, Dublin. During the two years, 1950-52, devoted to an MA in Modern History from UCD, he found inspiration in the teaching of Desmond Williams, and spent two semesters at Bonn University, Germany.
While still a student, Fennell contributed a column in Irish to The Sunday Press. There he befriended Douglas Gageby who, later, as editor of The Irish Times, was to give him free rein in that newspaper. Back in Germany in 1955 as an English newsreader on Die Deutsche Welle (German overseas radio), he contributed articles to Comhar and The Irish Times; radio talks to writer Francis McManus at Radio Éireann; and theatre criticism to The Times, London.
Immersion in German culture had aroused in Fennell an interest in the human condition. This in part explains a feature of his early writing — it would surface again in the 1990s — which differed from the practice of Irish Catholics generally. He investigated and wrote about “alien” - non-Irish, non-Catholic - lands and peoples. His first book Mainly in Wonder (1959), published by Hutchinson, London, was a reflective account of a year’s travel mainly in the Far East. After a year as first Aer Lingus sales manager for Germany, he spent 1960 researching a book in what was then avant-garde “pagan” Sweden, and contributed to The Irish Times the first direct reportage from the Soviet Union (15 articles) to appear in an Irish newspaper. He returned to Ireland in 1961 and summarised his Swedish experience in an essay `Goodbye to Summer' (The Spectator, London, 9 February 1962) which drew press reaction from Sweden to the US
Fennell, for whom painting has always been a passion, wrote art criticism for several Dublin publications. At the same time, partly influenced by the approaching fiftieth anniversary of the 1916 Rising, he read the literature of the Irish Revolution, identifying its core as a restorative humanism. Significant essays of this time were `The Failure of the Irish Revolution - and Its Success', `Cuireadh chun na Tríú Réabhlóide' and `Irish Catholics and Freedom since 1916'. The completion of the Revolution’s aims of Irish intellectual autonomy and democratic self-government became a guiding motivation and theme of his writing for the next thirty years.
In 1964 he moved with wife and son to Freiburg, Germany, as assistant editor of Herder Correspondence, the English-language version of Herder-Korrespondenz; a Catholic journal of theology, philosophy and politics which played a leading ‘progressive’ role during the Second Vatican Council. In 1966, as editor, Fennell returned to Dublin. Two years later he resigned and moved with his family to Maoinis in Irish-speaking Connemara.
During the following four years Fennell wrote a column for the Dublin Sunday Press. His principal themes in the Connemara period (1968-1979) were the “revolution” of the Gaeltacht or Irish-speaking districts (in which he participated); the pursuit of a settlement in Northern Ireland at war; decentralisation of Irish government to regions and districts; and criticism of the cultural thrust of the new-style liberalism, including historical revisionism, which was rising to ascendancy in the Dublin media and the universities.
Mainly in The Irish Times, The Sunday Press and several pamphlets, he modified the nationalist position on the North in ways which furthered movement towards the settlement of the 1990s. While pioneering recognition of the Northern unionists as British - 'the Ulster British' - he argued for British-Irish joint rule, persuaded the SDLP to declare for it, and supported Sinn Féin's four-province federal proposal.
From 1976 to 1982, Fennell lectured in Political Science and tutored in Modern History at University College Galway. In 1982 he returned to Dublin as lecturer in English Writing in the Dublin Institute of Technology. From the following year, and through the 1990s, a succession of books appeared, gradually shifting focus from mainly Irish themes to the contemporary West and European history (see titles below). In 1990, the National University of Ireland awarded him its DLitt (Doctor of Literature) degree for his published work.
In that same year he had begun his second “abroad” period with a visit to East Germany to record the last days of that Communist state in Dreams of Oranges. In 1993, when he retired from his lecturing post, a month in Minsk, Bielorussia, was followed by a six-weeks holiday in the USA during which he realised that the contemporary West was "postwestern" and began to have a new view of European history. In 1995, pursuing these insights, he returned to the USA and spent 15 months in Seattle. Since 1966 those two themes have been the predominant ones in his books (details of these at External link) Fennell. Some of these were written in Anguillara Sabazianear Rome, where Fennell resided from 1997-2007. In the latter year he returned to Ireland and has been publishing new essays on his website www.desmondfennell.com/