At certain low tides it can still be reached on foot from the mainland, a detail that would not have been lost on whoever chose the spot. The island's Irish name, Oileán Lócháin, is itself an open question: it may refer to a saint called Lochán, two of whom appear in the Martyrology of Oengus, a text written around AD 800, or it may simply mean the island of the chaff. No historical documentation survives to settle the matter.

Four seasons of archaeological excavation between 1992 and 1995 uncovered roughly 70 per cent of the island's surface, and what emerged was a layered sequence of devotional architecture built one upon another in a remarkably confined space. The earliest structure identified was a sod oratory, a small rectangular chapel built not from stone but from walls of compacted turf braced with rows of upright sandstone slabs. Walls of this kind were prone to subsidence and collapse, and the excavators found evidence that the southern wall had been rebuilt at least once. None of the turf blocks survived, but the trench patterns and a surviving sill stone, the threshold of the original doorway, allowed the dimensions to be partially reconstructed: an internal width of around 2.0 metres and a length that was considerably greater than what came after it. Five graves to the east of the structure confirmed its function as a place of worship rather than a dwelling. Over this sod oratory, later builders constructed a drystone oratory, an oratory being a small private chapel, built without mortar from carefully fitted stone, and so close in width to its predecessor (2.2 metres internally) that only traces of the earlier trench survived beneath its northern wall. A radiocarbon date derived from a layer above the clay floor of this later stone structure places activity there between roughly 640 and 790 AD. Also associated with the site is a leacht, a type of commemorative cairn or structured heap of stones used in early Irish monasticism for prayer and veneration, found on the north side of the drystone oratory's foundations.

The island sits in the civil parish of Killemlagh in the barony of Iveragh, a stretch of the Kerry coast already known for its early monastic remains. The proximity of a similarly named ecclesiastical site, Killoughane (Cill Lócháin), at the eastern end of the Iveragh peninsula, lends some weight to the theory that Illaunloughan took its name from the same obscure saint, though the connection remains circumstantial. What is certain is that on a patch of ground barely a tenth of a hectare in area, successive generations rebuilt, repaired, and continued a tradition of religious life that left its outline, quite literally, pressed into the earth beneath the island's surface.