That just sounds like being an ass for shits and giggles.
Oh, he is, believe me. But the thing is, he's a clever ass; he know what he can and can't say, and deliberately pushes the boundaries of decency and taste just to get a reaction. He means very little, if any, of what he says.
The thing is, I personally don't think accepting stereotypes - be they ethnical, racial, etc. - is that easy or will make them magically "go away". People will always say hurtful things for whatever reasons. Taking it as a joke and sweeping it under the proverbial carpet doesn't change the fact that they're often done to be cruel and offensive. As far as I'm concerned, there's a line in comedy you don't cross, and I wouldn't blame people for feeling uneasy around those kind of jokes.
But to get back to the original point, I just think the whole "Irish people talk like leprechauns" schtick makes them come across as ignorant. If you're going to make a joke about being Irish, at least come up with something more original.