Allgood is one of eight children of compositor George and french polisher Margaret (née Harold) Allgood. Her father is sternly Protestant and against all music, dancing and entertainment, while her mother is a strict Catholic. After her father dies in 1896, she is placed in an orphanage. She is apprenticed to a dressmaker and her brother Tom becomes a Catholic priest.
Maud Gonne sets up Inghinidhe na hÉireann (Daughters of Ireland) in 1900 to educate women about Irish history, language and the arts, and Allgood and her sister Sara join the association’s drama classes around 1903. Their acting teacher, William “Willie” Fay, enrolls them in the National Theatre Society, later known as the Abbey Theatre. Allgood is part of the Abbey Theatre from 1906-1918 where she appears in many productions. In 1904 she is cast in a play by Irish playwright Teresa Deevy called Katie Roche where she plays the part of Margaret Drybone. There are 38 performances in this production.
In 1905 Allgood meets Irish playwright John Millington Synge and they fall in love, a relationship regarded as scandalous because it crosses the class barriers of the time. In September 1907 he has surgery for the removal of troublesome neck glands, but a later tumour is found to be inoperable. They become engaged before his death in March 1909. Synge writes the plays The Playboy of the Western World and Deirdre of the Sorrows for Allgood.
In June 1911 Allgood marries G. H. Mair, drama critic of the Manchester Guardian, and later Assistant Secretary of the British Department of Information, Assistant Director of the League of Nations Secretariat in Geneva, and head of the League of Nations office in London, with whom she has two children. He dies suddenly on January 3, 1926. Six months later she marries Arthur Sinclair, an Abbey actor. They have two children but the marriage ends in divorce.
Under her professional name Maire O’Neill, Allgood appears in films from 1930-53, including Alfred Hitchcock‘s film version of Seán O’Casey‘s play Juno and the Paycock (1930). She makes her American debut in New York City in 1914 in the play General John Regan at the Hudson Theatre.
Joseph O’Connor‘s 2010 novel, Ghost Light, is loosely based on Allgood’s relationship with Synge.