Lying in Clew Bay to the south-west of Achill, it has long been associated with early Christian monasticism and with the Croagh Patrick pilgrimage tradition, functioning as a stopping point for those travelling by sea to the mountain. The island's name, derived from the Irish word for a stone fort, hints at the layered occupation that characterises so many of these Atlantic fragments.

The remains on Caher Island include an early monastic enclosure, a small oratory, and a series of cross-inscribed slabs and bullaun stones, the latter being shallow basin-like depressions ground into rock, used historically for grinding grain or, in ritual contexts, for collecting water believed to have curative properties. The site is associated with Saint Patrick, though this association, as with much of early Irish hagiography, reflects later tradition as much as historical record. What is less disputable is the physical evidence of sustained early medieval use, including the enclosing wall that defines the ecclesiastical precinct and the carved stonework that speaks to a community with both the time and the inclination for careful devotional labour.

The island is accessible only by boat, and crossings are weather-dependent in a part of the Irish coast that does not always cooperate. The pilgrimage pattern, a barefoot circuit of the island's sacred stations, is still observed by some visitors, particularly around the feast day of Saint Patrick in late summer in the local tradition rather than in March. The cross slabs are worn but legible, and the enclosure wall, though ruinous in places, gives a clear sense of the original boundary between the sacred interior and the open Atlantic beyond it.