Tynan was born into a large farming family in Clondalkin, County Dublin, and educated at a convent school in Drogheda. Her poems were first published in 1878. She met and became friendly with the poet Gerard Manley Hopkins in 1886. Tynan went on to play a major part in Dublin literary circles, until she married and moved to England; later she lived at Claremorris, County Mayo when her husband was a magistrate there from 1914 until 1919.

For a while, Tynan was a close associate of William Butler Yeats (who may have proposed marriage and been rejected, around 1885), and later a correspondent of Francis Ledwidge. She is said to have written over 100 novels; there were some unsurprising comments about a lack of self-criticism in her output. Her Collected Poems appeared in 1930; she also wrote five autobiographical volumes.

Tynan died in 1931 in Kensal Green, London.