Image thanks to Frank … the story, thanks to Wiki:
St. Patrick banishes all snakes from Ireland
Pious legend credits St. Patrick with banishing snakes from the island, 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 mythography of the staff of Moses, messenger of Yahweh to gentile Egyptians. 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.
However, all evidence suggests that post-glacial Ireland never had snakes, as on insular “Ireland, New Zealand, Iceland, Greenland and Antarctica…So far, no serpent has successfully migrated across the open ocean to a new terrestrial home” such as from Scotland on the neighboring island of Britain, where a few native species have lived, “the venomous adder, the grass snake, and the smooth snake”, as National Geographic notes, and although sea snake species separately exist. “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. The List of reptiles of Ireland has only one land reptile species native to Ireland; the viviparous or common lizard.
One suggestion, by fiction author Betty Rhodes, is that “snakes” referred to the serpent symbolism of the Druids during that time and place, as evinced on coins minted in Gaul. Chris Weigant connects “big tattoos of snakes” on Druids’ arms as “Irish schoolchildren are taught” with the way in which, in the legend of St. Patrick banishing snakes; the “story goes to the core of Patrick’s sainthood and his core mission in Ireland.”