It is a small, practical intervention, but it speaks to a long afterlife of use and adaptation, and it is the kind of detail that rewards anyone paying close attention to the fabric of the building.

The church is a nave and chancel type, one of the most common forms found in medieval Irish ecclesiastical architecture, where a rectangular nave for the congregation connects to a smaller chancel at the east end reserved for the clergy. Here, the two sections are divided by a round chancel arch, a feature that tends to suggest earlier medieval origins. The walls are built of roughly coursed limestone blocks, and the nave measures twelve metres in length and six metres in width, with walls around 0.85 metres thick. Opposing doorways at the western end of the nave have slightly pointed segmental arches, and the windows throughout are plain, with splayed embrasures, the angled inner reveals that widen the spread of light into the interior. According to Healy's 1975 survey, windows appear in the west wall, the north wall, and two in the south wall. The interior has at some stage been used for interments, which was not uncommon in Irish churches long after formal liturgical use had ceased.

The church sits within a graveyard, and like many such sites across rural Ireland, it has been subject to well-meaning but problematic repairs over the years. Ribbon pointing, where a raised bead of mortar is applied over the joints between stones rather than flush or slightly recessed, is visible on the exterior; conservationists generally consider this damaging to old masonry because it traps moisture and accelerates decay. Vegetation is re-establishing itself around the structure, and there is mortar wash-out along the base of the walls, signs that the building requires ongoing attention. Killeek itself sits in north Dublin, not far from the airport, and the graveyard is the kind of place where the surrounding suburban and industrial landscape makes the survival of such a structure feel all the more incongruous.