St Patrick's Purgatory is an ancient pilgrimage site on Station Island in Lough Derg, County Donegal, Ireland. According to legend, the site dates from the fifth century, when Christ showed Saint Patrick a cave, sometimes referred to as a pit or a well, on Station Island that was an entrance to Purgatory. Its importance in medieval times is clear from the fact that it is mentioned in texts from as early as 1185 and shown on maps from all over Europe as early as the fifteenth century. It is the only Irish site designated on Martin Behaim's world map of 1492.
Location
In the nineteenth century there was some confusion about the actual site of St Patrick's Purgatory—whether it was on Station Island or Saints Island on Lough Derg, County Donegal. For instance the early nineteenth-century Ordnance Survey maps of Ireland (1837–42) locate the Purgatory on Saints Island. However, its location on Station Island is a tradition that continues unbroken from the Middle Ages. It is clearly indicated on documents dating from that time, and it appears as "Caverna Purgatory" on the detailed map of Station Island in Sir James Ware's De Hibernia (1654) and Fr. Thomas Carve's book, Lyra Hibernica (1666).
Foundation
Legend maintains that St. Patrick had grown discouraged by the doubts of his potential converts, who told him they would not believe his teachings until they had substantial proof. St. Patrick prayed that God would help him relate the Word of God and convert the Irish people, and in return, God revealed to him a pit in the ground, which he called Purgatory; by showing this place to the people, they would believe all that he said. By witnessing Purgatory, the people would finally know the reality of the joys of heaven and the torments of hell.
Given the sparsity of any documentation for fifth-century Ireland, it is not surprising that there is no proof that St. Patrick ever visited Lough Derg. And while this is the legend, it is a rather late legend dating probably from the twelfth century. There is however a much more firmly established tradition regarding St. Dabheog or Dabheoc, a local abbot who presided over, and possibly established, the monastery on the site during the lifetime of Patrick. His name has been associated from these early centuries with several places in the area, for instance: St. Dabheoc's Chair on the south bank of Lough Derg, the townland of Seedavoc (St. Dabheoc's Seat), and a mountain in that townland, Seadavog Mountain. One of the islands in Lough Derg was also named after him: St. Dabheoc's Island, which may have been Saints Island or another island entirely.
Cave
Although the cave has been closed since 25 October 1632, several descriptions by early pilgrims survive. They referred to it as a cave or cellar or as an enclosed pit. The entrance, which was kept closed and locked, was quite narrow: about 0.6 m (2 ft) wide and 0.9 m (3 ft) high. Once inside there was a short descent of about six steps. The cave was divided into two parts: the first was about 3 m (9 ft) long, probably with banked sides and only high enough to kneel in; after a turn there was another niche about 1.5 m (5 ft) long.
Since the site has never been excavated, we can only rely at this point on these descriptions of the cave. However based on other archaeological excavations it seems clear that this was probably an ancient structure. Some have suggested a souterrain, a place for storing crops and animals. However, the size of the cave would make this seem very unlikely. A much more plausible suggestion is that it was one of the ancient sweat houses, which were actually still in use in Ireland into the twentieth century. From modern practice we know that people would enter these small enclosed places to inhale medicinal smoke produced by burning various plants. The name "purgatorium" could possibly have been used here originally with its Latin meaning as a place for cleansing and purging—much like a modern sauna; especially since the modern notion of "purgatory" as a place for punishment in the afterlife did not come into common use until the thirteenth century. The cave would then have been a place that people went to for physical or spiritual healing, even before it became associated with St. Patrick in the twelfth century as a place for strictly spiritual healing.
Monastery and its functions
A monastery probably existed on the islands in Lough Derg from the fifth century and it probably included anchorites who lived in beehive cells—which may be preserved in some form in the penitential beds that can still be seen on Station Island.