It can also be used to describe anyone who despite many generations in the new world and questionable Irish ancestry constantly proclaims themselves to be Irish. This can range from an innocent baseball cap with a shamrock on it to a drunken twit in a kilt idiotically greeting everyone with a faux accented "top o' the mornin' to yah laddie".
Robert : Yeah I see. It's great little costume he has there you'd almost think the oirish twit was auditioning for a part on Ballykissangel.
American Tourist : Put a Shamrock in Me Guinness.
Bartender : ::Tosser::