Why does he always play losers/drunks? Irish Stereotype?

Of course he's like this in real life.

Unfortunately the world loves to make fun of the Irish by portraying them as drunks. It's quite sad, when you consider how beloved the Irish are and everyone claims to be Irish just because "great great great great Grandfather O'Neill emigrated from Ireland when he was a fetus!"

His most famous Irish movie, Michael Collins, he plays a very important man in Irish history (Michael Collins, 'the big fella') and is a force to be reckoned with, and his only seen drinking once but is not drunk. So there's something. Michael Collins was not a drunk.

He wasn't a drunk/loser in Schindler's List, Nell, Taken, Chronicles of Narnia/Lego Movie/Ponyo (even though it was just a voice part), Star Wars Episode One, Batman Begins, Clash of the Titans, Kingdom of Heaven, Kinsey, The Haunting, Michael Collins and Les Miserables.

I'm not saying that all the above mentioned movies are great (especially Star Wars Episode One), but I think they show his versatility.

Probably so his character can go through some Redemption plot.