When I was in college, I had a friend who was REALLY into her Welsh heritage (she could recite from memory her entire Welsh name, which is apparently all the first names of your ancestors added on to your own). She informed me (amongst many other things) that the Welsh were also Celtic in origin, like the Irish. So many of the pagan traditions were the same. I happened to talk to her on St. Patrick's Day one year, and she informed me of the pagan Celtic origins of the holiday. "I'm never wearing green on St. Patrick's Day! Only on my wedding day," she told me. Why? In pagan tradition, women wore green to basically announce that they were ready to "become a woman." (kind of like a "coming out" party but more sexual in nature).
Now, as most people know, the Catholics were fond of trying to convert the pagans by "Christinizing" their festivals/holidays and making them in to Christian ones. So, when they were horrified by the sexual nature of this particular pagan custom, they decided to make it into a holiday to honor St. Patrick (who was famous for driving all the snakes out of Ireland--the "snakes" were actually druids, btw). They twisted the meaning around, so that the pagans could still wear green but totally changed the meaning of the holiday--that wearing green kept people away (in modern times to stop "pinching").
Ironically, most people in Ireland don't even celebrate it. It's a much bigger holiday here in the U.S. I believe for 2 reasons: 1) for those with Irish ancestry to honor their heritage and 2) for everyone (Irish or not) to get drunk.