Irish Americans really aren't that "ethnic." They've been in the US longer, came from the British Isles and spoke English. And you don't need to cast Irish American actors to play Irish American characters as in the case with Italian actors and characters, any generic white American (or even Brit who can do a good American accent) will do.
From what I've seen the Brits generally do a bang up job of impersonating mobsters. See Gary Oldman in
State of Grace. Easily one of the best gangster impersonations I've ever seen.
