According to tradition, St. Patrick founded a monastic settlement here in the fifth century, appointing Dabheoc as its first abbot. Whilst forty-six islands dot the lough, only two hold archaeological interest, with Saint’s Island (formerly St. Dabheog’s Island) believed to be the original site of the monastery. This settlement later became a priory under the Augustinian Abbey of SS Peter and Paul in Armagh during the 1130s, before being suppressed and abandoned at the end of the sixteenth century. The monastery’s primary purpose was to serve pilgrims visiting St. Patrick’s Purgatory, a cave that may have originally been located on Saint’s Island before being relocated to nearby Station Island.

The Lough Derg pilgrimage gained fame throughout medieval Europe, spawning an extensive body of literature and drawing countless visitors along an ancient roadway from Pettigo village. This route passes notable sites including Rathnacross ringfort and Templecarn old church and burial ground, continuing to the lake’s southwest shore where a wooden bridge once connected to Saint’s Island. Natural boulders protruding from the water may be remnants of this bridge’s supports. The area also features several religious landmarks: St. Brigid’s Chair, a natural L-shaped stone on the southeast shore, and St. Dabheoc’s Seat, which once consisted of a stone seat before a grave-like opening atop a southern hill, though forest growth has since obscured these remains.

Archaeological surveys of Saint’s Island reveal a complex of monastic structures, including the western half of a single-banked earthen enclosure approximately 20 metres in diameter with an exterior fosse up to 1.5 metres wide. The eastern section appears to have been destroyed by a later graveyard, a rectangular space measuring 27 by 22.5 metres enclosed by drystone walls. At its centre lies an irregular pile of rubble scattered with graveslabs, whilst a 44-metre pathway marked by two lines of grass-grown stones leads northeast to the foundations of another rectangular structure measuring 9 by 6.55 metres. Additional features include part of what appears to be a circular enclosure with walls over 18 metres in diameter, various earthen banks and rubble heaps indicating old field boundaries, and evidence of an ancient quay or landing place at the island’s northeast end.