Of the forty-six islands scattered across the lake, two stand out for their archaeological significance: Saint’s Island, which housed the original monastery, and Station Island, home to the famous pilgrimage site. The monastery on Saint’s Island evolved into an Augustinian priory in the 1130s, serving as a dependency of the Abbey of SS Peter and Paul in Armagh, before its eventual suppression and abandonment in the late sixteenth century. Its primary role was ministering to the countless pilgrims who journeyed to St. Patrick’s Purgatory, a cave that some scholars believe originally existed on Saint’s Island before the pilgrimage relocated to Station Island.

The medieval pilgrimage to Lough Derg gained such renown throughout Europe that it spawned an extensive body of literature, drawing visitors from far beyond Ireland’s shores. The ancient pilgrimage route from Pettigo village still bears traces of this sacred journey, passing by Rathnacross ringfort and the old church and burial ground at Templecarn. Where the path reaches the southwest shore of the lake, remnants of a wooden bridge’s natural boulder supports mark the crossing point to Saint’s Island. The landscape holds other sacred markers too; St. Brigid’s Chair, a naturally formed L-shaped stone, sits on the southeast shore, whilst St. Dabheoc’s Seat once crowned a hill to the south, though forestry has now obscured this site.

Today, Station Island presents a dramatically different face, with a twentieth-century basilica and administrative buildings covering most of its surface. The penal beds, circular structures roughly 2.8 metres across that resemble clochán foundations, appear to be relatively recent additions despite their ancient appearance. Among the island’s ecclesiastical treasures are several notable artefacts: St. Patrick’s Cross, a 1.3-metre cylindrical shaft decorated with spiralling bands, reportedly transferred from Saint’s Island; a broken cross head measuring 54 centimetres wide with a central depression encircled by a raised band, now built into a modern wall; and a small font bearing an inscription that nineteenth-century scholars read as “MANUS COIVANI”. These fragments of stone and scripture serve as tangible links to centuries of devotion, prayer, and pilgrimage that have shaped this sacred landscape.