Since I am mostly Irish I figured I would provide a link to the the Yahoo! article explaining some of the Irish myths surrounding St Patrick's Day.
One is St Patrick was not born in Ireland, which I thought was common knowledge. But an interesting thing that makes sense, although I never thought of it, was that St Patrick never drove the snakes out of Ireland. They were never there in the first place. Evolutionarily speaking this makes perfect sense, but again I had never thought of it before. The other stuff is pretty interesting as well and you should take a gander at it.