Of the forty-six islands dotting the lough, Saint’s Island (once known as St. Dabheog’s Island) housed the original monastery, which later evolved into an Augustinian priory under the control of SS Peter and Paul Abbey in Armagh during the 1130s. The priory’s primary purpose was to serve the countless pilgrims who journeyed to St. Patrick’s Purgatory, a cave that drew visitors from across medieval Europe and inspired a substantial body of literature. Though the cave is now located on Station Island, some historians believe it may have originally been situated on Saint’s Island before the pilgrimage site was relocated.

The medieval pilgrimage route to Lough Derg can still be traced today, winding from Pettigo village past several archaeological sites including Rathnacross ringfort and Templecarn’s old church and burial ground. This ancient roadway led travellers to the southwestern shore of the lake, where a wooden bridge once connected the mainland to Saint’s Island; natural boulders protruding from the water may be remnants of the bridge’s original supports. The priory itself was suppressed and abandoned by the late sixteenth century, leaving behind only traces of its once-thriving religious community.

The landscape around Lough Derg preserves several intriguing features linked to its sacred past. On the southeastern shore sits St. Brigid’s Chair, a naturally formed L-shaped stone, whilst St. Dabheoc’s Seat once occupied a hilltop south of the lake, consisting of a stone seat positioned before what early observers described as a ‘grave-like opening’. These physical remnants, combined with the enduring pilgrimage tradition on Station Island, ensure that Lough Derg remains one of Ireland’s most significant religious landscapes, where centuries of devotion have left their mark on both the islands and the surrounding countryside.