His breakthrough role came when he was cast as Gar Private in the world premiere of Brian Friel's "Philadelphia, Here I Come!" directed by Hilton Edwards for the Gate Theatre at the Dublin Theatre Festival in 1964. The production subsequently transferred to Broadway where it played for over 300 performances and established Donnelly and Patrick Bedford – who played his alter-ego Gar Public – as formidable new talents to be reckoned with. They were jointly nominated for the Tony Award for Best Performance by a Leading Actor in a Play in 1966.