Most medieval churches were built to shelter congregations. The one at Killelan, in County Kildare, seems to have been built partly to repel them. What survives on a low rise within an old graveyard is a fortified tower, the kind of structure more readily associated with a lord's stronghold than a place of worship. Locals, it turns out, came to the same conclusion: the antiquarian John O'Donovan, recording the site in 1837, noted that the building was known simply as the Castle of Killelan.

The remains are substantial enough to reward close attention. The tower retains a high barrel vault, a form of continuous arched ceiling common in Irish ecclesiastical building, with large two-centred windows set into the east and south walls. At the west end, a piscina, the small stone basin used for draining water after the washing of sacred vessels, survives in the south wall, with a cupboard neatly opposite it. A circular turret projects from the north wall near the northwest angle, and within it a spiral staircase once gave access to an upper floor that no longer survives. Two small lintelled chambers sit on either side within the thickness of the vault, the kind of tight, purposeful spaces that suggest the building was designed with defence, or at least security, in mind. The foundations of a nave extend westward, hinting at a more complete church that has long since disappeared. O'Donovan believed the whole complex had once belonged to an abbey on the same site, though no trace of any such institution appears to have survived beyond local memory and the castle-like tower itself. The walls show clear signs of repair and rebuilding across the centuries, meaning what stands today is a palimpsest, layers of construction and reconstruction settled into a graveyard that is still in use.