Sometime in the late fifteenth or early sixteenth century, somebody inserted a stone vault into the existing church structure and stacked residential accommodation above it, converting what had been a place of worship into something resembling a tower house. A stone altar still standing in front of the east window suggests the church did not simply cease to be a church when this happened; the two uses appear to have coexisted, the vault overhead and the liturgy continuing below.

The original building dates to the late twelfth or thirteenth century, as indicated by the roll-moulded jambs of the west doorway and the proportions of the surviving window embrasures, which suggest tall narrow lancets of the kind typical of that period. The church was dedicated to St Brendan of Birr, according to the historian Carrigan writing in 1905, and a holy well dedicated to the same saint lies around two hundred metres to the northwest. The fabric of the building holds layer upon layer of alteration. Voussoirs, the wedge-shaped stones that form an arch, survive in several places above and around windows that were later blocked or replaced, making it possible to reconstruct something of the original fenestration even though none of the first windows remain intact. A pointed recess in the west wall at first-floor level has been interpreted by some as a pulpitum, the kind of elevated platform associated with cathedrals and abbey churches, though its presence in a parish church context is considered unlikely; it may instead relate to a chancel or rood screen, where a cross or statues would have been displayed. A shallow stoup set into a rough aumbry, a small wall recess used to store sacred vessels, survives at the east end of the south wall. The Office of Public Works carried out substantial repairs in the mid-1930s, covering the vault with concrete and inserting tie-bars, with further works following in 1952. Approximately twenty-five metres to the southwest, a round tower completes an unusually dense cluster of early medieval remains on this one modest rise of ground.