What survives at Castlefarm in County Kildare is a building that has been, in a sense, three different things on the same ground. Beneath the ivy-covered rubble of a medieval nave and chancel, there may lie the footprint of an early Christian monastery founded, according to a 1943 survey, by St. Briga in the fifth century, though nothing visible now confirms it. The ruins standing today belong to a later age, and they are sparse enough: sections of the north and south sidewalls of a nave measuring roughly twelve and a half metres long and just under six metres wide, built in undressed rubble masonry, and an east gable wall that still frames a chancel arch two and a half metres wide and nearly three metres high. Of the chancel itself, only its east gable, with a robbed-out window opening, and short stubs of its sidewalls remain. A stone font survives inside among the burials.

The ecclesiastical history of the place runs in two distinct chapters. The early foundation attributed to St. Briga left no physical trace, but the site was revived in a quite different form when a cell of the Augustinian Cartmel Priory in Lancashire was established at Kilrush at or before 1201 to 1202, the date by which Cartmel was already receiving grain from Ireland. This was not a full conventual priory, the kind of house with a complete community living under a formal rule, but rather a working farm staffed by three or four canons, functioning primarily to supply the English mother house with agricultural produce. In 1558, following the dissolution of the monasteries, Kilrush was granted to the Earl of Ormond. The grant at that point included a castle, a number of messuages (dwellings with their outbuildings and yards), and 360 acres of arable land, suggesting that whatever spiritual life had persisted here had long been subsumed into an estate economy.

The ruins sit within a graveyard that is still in use, and burials continue to occupy the interior of the roofless shell. The chancel arch, one of the more legible features remaining, gives some sense of the building's original proportions, even as the ivy and the general collapse of the walls make it difficult to read the structure clearly. The font, still in place inside, is an unexpected survival given how thoroughly the rest of the fabric has been stripped or tumbled.