An Irish film and theatre actor, he is often noted by critics for his chameleonic performances in diverse roles and distinctive blue eyes.

He starred in Irish and UK film and stage productions throughout the late 1990s and early 2000s, first coming to international attention in 2003 as the hero in the post-apocalyptic film 28 Days Later. His best-known roles are as villains in two 2005 blockbusters: the Scarecrow in Batman Begins, and Jackson Rippner in the thriller Red Eye. Next came two contrasting, widely acclaimed starring roles: as transgender outcast Kitten in Breakfast on Pluto (2005) and as a 1920s Irish revolutionary in The Wind That Shakes the Barley (2006). In 2010, he co-starred in Inception.