What does the snake have anything to do with Saint Patrick?

The legend was that St. Patrick drove all the snakes out of Ireland may have a more literal meaning to it. Ireland at the time was Celtic Druid and not Christian. St. Patrick was very good at converting the Irish people to Christianity. One of the Druid symbols were a snake. So by converting the Irish people to Christianity, he in essence drove out the snakes (Druids out of Ireland).