O’Neill is born Mary Devenport, the daughter of Royal Irish Constabulary (RIC) sub-constable, John Devenport, and his wife Bridget (née Burke). She attends the Dominican convent, Eccles Street, Dublin before enrolling in the Metropolitan School of Art from 1898 to 1903. In 1900 she wins the year’s prize in the School of Art. She apparently considers teaching as a career, as she is listed on the college register as a teacher in training from 1901 to 1903. It is while an art student that she starts to correspond with the writer she admires, Joseph O’Neill. Their relationship develops, and the couple marries on June 19, 1908, settling in Kenilworth Square, Dublin.
Many of O’Neill’s husband’s friends disapprove of her modern and unconventional ideas, but she is popular with “the Rathgar Group” who attends George Russell’s Sunday salons. After a few years, she establishes her own salon referred to as “Thursdays at home,” attended by Russell, Padraic Colum, W. B. Yeats, Richard Irvine Best, Frank O’Connor, Francis Stuart and Iseult Gonne. She becomes particularly close to Yeats, who she confides in. Yeats records their weekly consultations in his diary while working on A Vision (1925). In his Oxford anthology of English verse from 1936, he includes one of O’Neill’s poems. In 1917, she contributes lyrics to her husband’s play The Kingdom Maker. She publishes her only book in 1929, Prometheus and other poems. After this she occasionally contributes primarily modernist plays and poetry to The Dublin Magazine, The Irish Times and The Bell. She collaborates with Austin Clarke from the Lyric Theatre Company on her plays Bluebeard (1933) and Cain (1945).
O’Neill suffers with poor health, which sees her and her husband spending extended periods in the south of France and Switzerland. They sell their home in Dublin in August 1950 and move to Nice, with the intention of settling there. However, due to rapidly depleting finances they are forced to return to Ireland in April 1951. They rent a cottage in Wicklow from their friend Con Curran. When her husband dies in 1953, she goes to live with relatives in Dublin. She dies there in 1967.