I always figured that St. Patrick's Day was probably just a normal Saints day in Ireland (after all, who's heard of St. George's Day in England or St. Denis's Day in France). Since most of us Americans don't really know anything, we can make it whatever we want.

There are no four-leaf clovers in Ireland primarily because the Fir'Bolg took them all when they left, just to annoy the hell out of everyone else.