A few of my own "great-grand-pawpaws" did come from Ireland, but I agree, it's weird to base your heritage on stuff which happened 100 years ago (or more) in a country you never visit, in most of these cases! I'm only Scottish.
Folk should just be honest and say they like St Pats because it gives them a chance to be drunk as they believe in stereotyping nations.
Mind you, probably just as well it's only the drinking stereotype they pick to emulate...
From experience, people mostly use it as an opportunity to get drunk.
In other words, in America, it's considered St Patricks Day, but in Glasgow, it was Friday.