Among its monuments is a cashel, a type of ringfort built from dry-stone walling rather than earthen banks, whose circular enclosure once served as a defended farmstead or the seat of a local lord. That such a structure survives on so remote an island is quietly remarkable; cashels of this kind were typically the domain of prosperous farming families in early Christian Ireland, roughly between the sixth and tenth centuries, and finding one here points to a period when Inis Gluaire was not the abandoned margin it might seem today, but a functioning part of an Atlantic world connected by sea rather than road.

Inis Gluaire is perhaps best known in early Irish literature as the island where the Children of Lir, transformed into swans, finally came to rest, and where the monk Brendan, associated with extraordinary oceanic voyages, is said to have founded a community. Whether or not those traditions map onto verifiable archaeology, they reflect the island's genuine reputation as a place of early Christian settlement. A cashel in this context would not be out of place alongside an ecclesiastical site; secular and religious enclosures were often built in close proximity in early medieval Ireland, and the island's isolation would have made it attractive both to hermits and to small farming communities seeking natural protection on all sides.