Mary Devenport O'Neill (3 August 1879 – 1967) was an Irish poet and dramatist and a friend and colleague of W. B. Yeats, George Russell, and Austin Clarke.

[1] Many of her husband's friends disapproved of her modern and unconventional ideas, but she was popular with "the Rathgar Group" who attended George Russell's Sunday salons.

After a few years, O'Neill established 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.

After this she occasionally contributed primarily modernist plays and poetry to The Dublin Magazine, The Irish Times and The Bell.

[1][4] O'Neill suffered with poor health, which saw her and her husband spending extended periods in the south of France and Switzerland.