One night a group of thugs attacked him as he left a pub, and tried to rob him. He attempted to fight back and was stabbed in the process, and from this came his facial scar; it's known as a Glasgow Grin.
He could have died, but he survived and chose to become an actor. He got his big break in the film Braveheart (1995) which starred Mel Gibson.
He's one of the stars of the TV series Sons of Anarchy, where he plays Chibs, and will be in Officer Down (2012), and Grace and Danger (2013).