From #Wikipedia : Patrick banishes all snakes from Ireland
The absence of snakes in Ireland gave rise to the legend that they had all been banished by St. Patrick chasing them into the sea after they attacked him during a 40-day fast he was undertaking on top of a hill. This hagiographic theme draws on the Biblical account of the staff of the prophet Moses. In Exodus 7:8–7:13, Moses and Aaron use their staffs in their struggle with Pharaoh's sorcerers, the staffs of each side morphing into snakes. Aaron's snake-staff prevails by consuming the other snakes.
However, all evidence suggests that post-glacial Ireland never had snakes. "At no time has there ever been any suggestion of snakes in Ireland, so [there was] nothing for St. Patrick to banish", says naturalist Nigel Monaghan, keeper of natural history at the National Museum of Ireland in Dublin, who has searched extensively through Irish fossil collections and records.