Patrick himself founded a monastic settlement here in the fifth century, appointing Dabheoc as its first abbot. Of the forty-six islands scattered across the lough, Saint's Island (formerly St. Dabheog's Island) is believed to be the site of this original monastery, which later became a priory under the Augustinian Abbey of SS Peter and Paul in Armagh during the 1130s. The monastery's primary role was serving 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 nearby Station Island, some sources suggest it may have originally been on Saint's Island itself before the pilgrimage route was relocated.

The archaeological remains on Saint's Island tell the story of centuries of religious activity. The western half of an earthen enclosure, roughly 20 metres in diameter with an exterior fosse, marks the earliest defensive structure. A rectangular graveyard measuring 27 by 22.5 metres, enclosed by a drystone wall, occupies much of the eastern portion where the original enclosure once stood. At its heart lies a jumble of rubble topped with graveslabs placed without apparent order, possibly the remains of a rectangular foundation shown on old Ordnance Survey maps. A 44-metre pathway lined with grass-covered stones leads northeast to another rectangular structure, measuring 9 by 6.55 metres, built with drystone walls that still stand nearly a metre high in places.

The island preserves other intriguing features that hint at its former importance as a pilgrimage destination. An ancient roadway from Pettigo village passes significant sites including Rathnacross ringfort and Templecarn church before reaching the southwest shore of the lough, where a wooden bridge once connected to Saint's Island; natural boulders jutting from the water may be remnants of its supports. Along the lough's shores, pilgrims would have encountered sacred landmarks such as St. Brigid's Chair, a naturally formed L-shaped stone on the southeast shore, and St. Dabheoc's Seat to the south, which once featured a stone seat before what was described in 1879 as a 'grave-like opening', though forestry has now obscured any traces of this site. The northeastern tip of the island shows evidence of an old quay or landing place, whilst various earthen banks and stone walls across the island appear to mark ancient field boundaries, painting a picture of a once-thriving religious community that served medieval Europe's spiritual seekers.