According to tradition, St. Patrick himself founded a monastic settlement here in the fifth century, appointing Dabheoc as its first abbot. The monastery’s primary role was tending to the countless pilgrims who journeyed to experience St. Patrick’s Purgatory, a cave that gained legendary status throughout Christendom. Whilst the pilgrimage site now sits on nearby Station Island, some historians believe the original cave was actually located on Saints Island before being relocated to its current position.

The ancient pilgrims’ route to this sacred site can still be traced in places, winding 8 kilometres from the village of Pettigo to the southwestern shore of the lake. This roadway, known as a via strata, passed significant landmarks including Rathnacross ringfort and Templecarn’s old church and burial ground before reaching the water’s edge, where a wooden bridge once connected the mainland to Saints Island. Natural boulders still visible in the lake may be remnants of the bridge’s original supports. When examined in 1879, investigators discovered that beneath accumulated peat and heath lay substantial evidence of this ancient causeway, including large stepping stones arranged in regular patterns, particularly visible on Portneillinmore hill and around Portcreevy bay.

Today, much of this historic route has been overtaken by forestry roads and modern development, though portions remain discernible to the keen observer. Near Carn, a section roughly three metres wide can still be seen, bounded by field walls and partially obscured by encroaching turf. The roadway hasn’t been properly used for two or three hundred years, allowing nature to reclaim much of its course, whilst waves have eroded the sections that ran along the lake shore. Despite these changes, the path stands as a tangible link to the time when Lough Derg served as one of Europe’s most important pilgrimage destinations, drawing the faithful to seek spiritual redemption in its remote waters.