"Everyone's Irish on St. Patrick's Day". I'm not Irish at all, but that's never mattered to me or anyone else on St. Patrick's Day! I'll be at the parade on Sunday only because it's nearby, and I'll wear green to be festive, but that's all. Actually, I'll probably do a little day drinking, too.
I celebrate Easter (i.e. chocolate bunnies and lots of food) and I'm not Catholic, and I celebrate Valentine's day by wearing red/pink and going out to dinner with my sweetie or whatever, I don't see St. Patrick's day as very different. It's origins are pretty meaningless at this point, it's juts a silly excuse to celebrate something with friends and family, to bring people together.