O'Shaughnessy's most notable stage role to date is that of Salome, for the Gate Theatre in Dublin. Other stage work has included The Shaughraun for the Abbey Theatre in Dublin, which transferred to the West End's Albery Theatre in 2005. In 2006 she appeared in the UK premiere of Blackwater Angel by Jim Nolan at the Finborough Theatre, London. Other work for the Gate Theatre includes Arms and the Man, Oliver, The Importance of Being Earnest, Pride and Prejudice, Blythe Spirit, Present Laughter (which toured in Charleston, South Carolina), See You Next Tuesday, and Cat on a Hot Tin Roof. She played Cate in the Irish debut of Sarah Kane's play Blasted.