I don't want people poking around in my private stuff. They've no business in it. My work is what I give to people, that's my job, and that's where it stops.

Joseph's CBS in Fairview Dublin where he was a member of the school drama group. While Gleeson portrayed Irish statesman Michael Collins in The Treaty he later portrayed Collins' close collaborator Liam Tobin in the film Michael Collins with Liam Neeson taking the role of Collins. Gleeson confirmed in July 2011 that he has secured funding for the project.

His best-known films include Braveheart Gangs of New York In Bruges 28 Days Later Troy the Harry Potter films The Guard and the role of Michael Collins in The Treaty. He won an Emmy Award in 2009 for his portrayal of Winston Churchill in the film Into the Storm.