Do you guys walk around wearing shirts that say "Kiss me I'm Irish" and "Luck of the Irish" and other things that reference your heritage A LOT or is it mostly an American thing?
No.i don't anyway and i don't know anyone that does. I dont think i have ever seen these t-shirts anywhere.

Anyways, Americans considers himself Italian if his grand grand grand grand² father was born in Italy.
i wear a shirt that says "smell me, im indian and my mom made some curry last night but didnt open the doors and windows so all the spices have gone into my shirts....i tried washing it, but downy fab. softener and then throwing some bounce dry sheets in it, but that didnt get it out... so yeah, i stink a little but nothing a little Burberry Touch cant fix...."
Hmm, I actually use the same cologne...

/\ Lol.

It's really an American thing...
Anyways, Americans considers himself Italian if his grand grand grand grand² father was born in Italy.
*the Republic of Ireland.Southern Ireland
*the Republic of Ireland.
It's not just an American thing. It happens to an extent over here on St. Patrick's Day.