Saint Patrick was a historical figure who had a very different place in Wizarding history than in Muggle history. His story was last taught by in History of Magic by Professor Anjelica Lainey during Term 18.
Unlike the Muggle version, in which Saint Patrick was a holy man who performed a number of miracles, including driving all the snakes from Ireland, the wizard Patrick was a Dark Wizard responsible for creating the first horcrux. Patrick placed his horcrux into the bodies of snakes, which explains the absence of snakes in Ireland. His house elves were tasked with the eternal job of guarding his horcruxes. The imagery of small, pointy-eared creatures guarding a mysterious treasure brought into being the Muggle tradition of leprechauns guarding pots of gold.