The island, known in Irish as Cathair na Naomh, or City of the Saints, contains the remains of an early monastic enclosure, a cashel wall, leacht or penitential stone monuments, and a number of crosses and cross-slabs that suggest a site of genuine ritual importance long before the medieval period. It lies on what was once a pilgrimage route associated with Croagh Patrick, and boats from the Roonagh Quay area would historically have carried pilgrims to the island as part of a wider pattern of devotional movement along that stretch of the Mayo coast.

The monuments on the island include a small oratory and several cross-marked stones, with the cashel, a roughly circular drystone enclosure wall, forming the structural core of the site. These enclosures were a characteristic feature of early Irish monasticism, defining sacred space in a landscape that had no parish boundaries in the modern sense. The leachta, low rectangular cairns used as stations in penitential circuits, indicate that Caher Island functioned not merely as a hermitage but as a place of structured devotional practice, probably drawing pilgrims across a long stretch of time from the early medieval period onward. The island is named after a saint whose precise identity has not been firmly established, though the density and character of the remains point clearly to a community or tradition of some duration.