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. 
A monastery  has been existed on the islands in Lough Derg from the fifth century.
Every year the main pilgrimage season begins in late May/early June and ends mid-August, on the 15th, the feast of the Assumption of Mary. It is a three-day pilgrimage open to pilgrims of all religions, or none, who must be at least fifteen years of age, in good health and able to walk and kneel unaided. Pilgrims, who should begin fasting at the previous midnight, assemble at the Visitor Centre on the shore of Lough Derg early in the day (between about 10 am and 1 pm). From there a boat ferries them on the brief trip out to Station Island. Once on the island they are assigned a dormitory room, and barefoot they begin a specified and almost continuous cycle of prayer and liturgies.
These prayers — the Our Father (or Lord's Prayer), the Hail Mary and the Apostles' Creed (all included in a booklet they receive)— are carried out at designated 'stations' on the island, including six 'beds' that are the remains of ancient cells or beehive huts, named for famous – principally Irish – saints. These are thought to be the remains of early monastic cells.
Pilgrims spend the first night in the island's basilica in prayer, and only on the second night can they finally sleep in the dormitory. Each day on the island the pilgrims have one simple meal of dry toast, oatcakes and black tea or coffee. On the third morning they are ferried back to the mainland, where they will continue their fast until midnight.
There are also other programs throughout the year, including Quiet Days, Family Days, and One-Day Retreats.
The site is under the stewardship of the Roman Catholic Diocese of Clogher. The staff includes people who can help with both spiritual and practical concerns. Facilities include a kitchen, laundry, first-aid station and book/gift shop.