I'm not sure he's actually done any movies where he uses his native Scottish accent anyway. So you've probably never heard him speak with it.
Scottish accents = Gerard Butler (when he's not in movies apparently), David Tennant, Ewan McGregor, Sean Connery (sort of), Ashley Jenson, Dougray Scott (despite putting on an awful English accent in Desperate Housewives!), James McAvoy
Irish accents = Colin Firth, Liam Neeson, Bono (and the rest of U2!)
The southern Irish accent is very distinctive. They don't say 'th', they say 't', so 'three' comes out as 'tree'. I used to love that. ;D The northern Irish accent is softer and a bit more similar to the West of Scotland accent (which is why people on many occassion have thought I'm Irish :P), but it's not too similar to the east/north of Scotland accents.