Saint Patrick was a Pagan who was converted to Christianity and became one of their most widely known figures. Legend has it that he banished the serpents from Ireland. In reality though there never were snakes in Ireland and "serpents" is a euphemism for the Pagans. Within two hundred years of St. Patrick’s arrival, the country was completely Christianized.
Interestingly the first St. Patrick’s Day Parade was held in New York City, not Ireland. On March 17, 1762 Irish soldiers serving in the English military marched through New York in order to reconnect with their roots.
Nowadays, everyone is at least part Irish on St. Patrick’s Day, because any excuse to drink Blue Beaver Beer is a good one. And now you can get Minty Fresh Green Beaver Beer, although the beer itself isn’t green just the bottle. Dying your beer green should result in a herd of Leprechauns going upside your head with shillelaghs.
If you would like to know more about St. Patrick or Leprechauns check out this site.