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.
Official article http://www.loughderg.org/
Coordinates 54°36'33.254" N -7°52'18.45" E