Whilst the lake contains forty-six islands, only two hold significant archaeological interest. The original monastery likely stood on Saint’s Island, which later became a priory under the Augustinian Abbey of SS Peter and Paul in Armagh during the 1130s, before being abandoned at the close of the sixteenth century. The monastery’s primary purpose was serving pilgrims who came to visit St. Patrick’s Purgatory, a cave that drew visitors from across medieval Europe and inspired a substantial body of literature. Some scholars suggest the cave originally existed on Saint’s Island before the pilgrimage relocated to Station Island, its current home.

Station Island today is almost entirely occupied by a twentieth-century basilica and administrative buildings, though remnants of its ancient past remain visible. The penal beds, circular structures roughly 2.8 metres in diameter, resemble clochán foundations but appear to be of more recent construction. Among the ecclesiastical fragments preserved here is St. Patrick’s Cross, a 1.3-metre cylindrical shaft decorated with spiralling bands, reportedly transferred from Saint’s Island. Other artefacts include the broken head of a cross 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 observers read as “MANUS COIVANI”.

The pilgrimage route itself tells its own story through the landscape. An ancient roadway from Pettigo village passes Rathnacross ringfort and Templecarn old church and burial ground before reaching the southwest shore of the lake, where a wooden bridge once connected to Saint’s Island; natural boulders jutting from the water may be the remains of its supports. Along the southeast 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 before what was described in 1879 as a “grave-like opening”, though forest growth has since obscured any traces of this site.