In 2001, at the age of 19, Condon originated the role of Mairead in "The Lieutenant of Inishmore" by Martin McDonagh which she performed at the Royal Shakespeare Company and in 2006 at the Lyceum Theatre in New York. For this production she recorded the song "The Patriot Game" with The Pogues. That same year, Condon played the role of Ophelia in "Hamlet", making her the youngest actress to ever play that role for the RSC. In 2009, she appeared in another play by Martin McDonagh, "The Cripple of Inishmaan", for which she won a Lucille Lortel award and a Drama Desk Award.