The lake contains forty-six islands, though only two hold significant archaeological interest. The original monastery likely stood on what’s now called Saint’s Island, which later became an Augustinian priory under the control of Armagh’s Abbey of SS Peter and Paul in the 1130s before being abandoned at the end of the sixteenth century. The monastery’s primary purpose was serving pilgrims journeying to St. Patrick’s Purgatory, a cave that drew visitors from across medieval Europe and inspired a substantial body of literature about the site.

Station Island, the current centre of pilgrimage, presents a striking contrast between ancient tradition and modern infrastructure. The island is dominated by a twentieth-century basilica and administrative buildings, but remnants of its religious past survive in various forms. The penal beds, circular structures roughly 2.8 metres in diameter that resemble clochán foundations, appear relatively recent despite their ancient appearance. Among the ecclesiastical fragments scattered across the island, St. Patrick’s Cross stands out; a 1.3-metre cylindrical shaft wrapped in spiralling bands of tracery, reportedly transferred from Saint’s Island. Other notable pieces include a broken cross head built into a modern wall, measuring 54 centimetres wide with a central depression encircled by a raised band, and a small font bearing an inscription that nineteenth-century scholars read as “MANUS COIVANI.”

The pilgrimage route itself tells a story of medieval devotion, with an ancient roadway leading from Pettigo village past Rathnacross ringfort and Templecarn church and burial ground to the lake’s southwestern shore. Here, a wooden bridge once connected the mainland to Saint’s Island, with natural boulders still visible in the water possibly marking where its supports stood. The landscape around Lough Derg preserves other curious features linked to religious tradition, including St. Brigid’s Chair, a naturally occurring L-shaped stone on the southeastern shore, and the former site of St. Dabheoc’s Seat atop a nearby hill, which once consisted of a stone seat before what observers described as a “grave-like opening,” though forest growth has since obscured any traces of this intriguing feature.