Saint Patrick was a 5th-century English bishop who is credited with converting Ireland to Christianity. He is so revered in Ireland that many legends grew up around his legacy, and it's hard to separate truth from fiction in some of them. One involves the entrance to Purgatory, on an island off the northwest coast of Ireland called Station Island. The concept of Purgatory was already established in the canon of the Catholic church, but it was considered a process instead of a place. St. Patrick, asking God for help in converting the Irish, was led to Station Island and to a cave, which is said to be the entrance to Purgatory. Anyone who entered would be confronted with the flames of judgement, and would be scared into conversion.
The story was first documented in 1184, hundreds of years after Patrick died. The book Treatise on St. Patrick’s Purgatory told the story of Patrick's discovery, and of a knight named Owen, who spent 24 hours locked in the cave and reported a thoroughly harrowing experience, sort of a preview of hell. The book became a bestseller across Europe, and brought thousands of pilgrims to Station Island. Eventually, the cave was filled in, and a basilica was built overtop. But it's still a pilgrimage site, although it's an unpleasant journey designed to test one's fortitude. Read about St. Patrick's Purgatory at Smithsonian.