The island is Illaunloughan, sitting in Portmagee Channel a short distance west of the village of Portmagee, and the structure at its north-eastern edge is a leacht, a type of commemorative or votive mound associated with early Irish Christianity, typically marking a grave or place of special religious significance. What makes this one unusual is that it still retains, in poor but legible condition, the remains of a gable-shrine: two large slate slabs, averaging around 1.3 metres by 1.1 metres, which formed the sides of a small roofed structure. One had slipped downward by the time archaeologists reached it. Beneath the shrine, excavation uncovered two small stone cists containing human bones, scallop shells, and quartz pebbles.

Four seasons of excavation between 1992 and 1995 exposed roughly seventy per cent of the island, and the picture that emerged was of a site dense with early medieval religious activity. The leacht itself is a substantial construction: a rectangular terraced platform measuring approximately 9 metres by 7.6 metres and rising to 1.5 metres, built partly over a levelled rock outcrop and retained along its edges by vertically set kerb-stones. Stone steps at its western side lead up to rough paving and then to the drystone structure carrying the shrine. Quartz stones, which carried strong symbolic associations in early Irish religious practice, were scattered liberally across the mound. As Marshall and Walsh noted in 1998, this type of gable-shrine is found at only a handful of sites on the western Iveragh Peninsula, including Killoluaig, Kilpeacan, and Killabuonia, and such shrines are generally thought to mark the grave of a monastic founder. The identity of whoever is commemorated here remains unknown. There is no surviving historical documentation for the island, and even its name is uncertain: Lócháin may refer to a saint mentioned twice in the Martyrology of Oengus, a calendar of Irish saints written around AD 800, or it may simply mean the island of the chaff.