Yvaine
I'm in ur circle! I'm killing ur druidz!!!11!!!
Okay, that was funny, but I do feel a major need to post in here.
The legend about St. Patrick "driving the snakes out of Ireland" is exactly that, a legend. There are no historical accounts of Pat actually killing these people. However, the OTHER thing he's famous for may shed some light on this. The reason the shamrock is such a symbol of St. Patrick and the Irish is because he used it in a lesson explaining the Holy Trinity, how there could be three but have all of them actually be the same one. Perhaps this helped him convert some of the Pagans (which would also kind of "drive them out", don't you think?)
As for the holiday in modern times, many people revel in it as an excuse to wear green (even if it looks horrible on them) and get absolutely hammered. Some people use it to celebrate Irish heritage (sadly, I'm mostly Scot/Brit). Heck, if you wanted to pull Pagans back into the holiday, you could think of it as an early start to Ostara or use the day to research or honor Irish gods.