She holds a place in theatre history as the first actress to interpret the lead character of Pegeen Mike Flaherty in John Millington Synge's controversial masterpiece The Playboy of the Western World (1907).

Born at 40 Middle Abbey Street, Dublin, O'Neill was one of eight children of compositor George and French polisher Margaret (née Harold) Allgood,[2][3] she was known as "Molly".

Maud Gonne set 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 joined the association's drama classes around 1903.

In 1905, Molly met Irish playwright John Millington Synge, and they fell in love, a relationship regarded as scandalous because it crossed the class barriers of the time.

She died in Park Prewett Hospital, Basingstoke, England on 2 November 1952, aged 66, where she was receiving treatment after being badly burned in a fire at her London home.