It is an uninhabited Atlantic island with a long history of early Christian use, and among its surviving monuments is a leacht, a term for a low, roughly rectangular stone cairn or altar associated with penitential pilgrimage practices. These structures served as stations where pilgrims would pray, often circling the cairn while reciting specific prayers, as part of a pattern, the Irish tradition of devotional rounds at sacred sites. The leacht on Caher Island sits within a wider complex of early medieval remains that suggests the island was once a place of some spiritual significance, likely connected to the broader network of island monasteries and pilgrimage routes along Ireland's western seaboard.

Caher Island, known in Irish as Cathair na Naomh, meaning something close to "city" or "stone fort of the saints", retains the ruins of an early oratory, enclosing walls, and several carved stones alongside the leacht. The island is associated in local tradition with St Patrick, though the surviving fabric of the site points more broadly to early Christian activity across several centuries. Pilgrimage to the island continued into relatively recent times, with pattern days drawing people across the water in small boats, a crossing that was never entirely without risk given the exposed conditions of Clew Bay and the open Atlantic approaches.