Most medieval churches in Ireland have a tower at one end or none at all. The ruined church at Mainham, Co. Kildare, has two, attached to opposite corners, and the remains of a third, older one pressed against the west gable. That accumulation of defensive stonework, added in stages over what appears to have been a considerable span of time, tells you that whoever held this place felt they needed to keep holding it, and to do so from a position of some strength.

The church itself is a plain limestone rectangle, roughly 19 metres long and 6 metres wide, built from small undressed blocks and flags laid in rough courses, with walls averaging over a metre thick. It is entered through a round-arched granite doorway with chamfered jambs in the south wall, which survives in good condition, though a 1985 restoration involved significant rebuilding and repointing that obscured a number of original features. Inside, all three of the original windows are blocked, both gable walls have been rebuilt without openings, and a concrete plinth now runs around the base of most walls. The better-preserved of the two main towers is attached to the south-east corner of the church, accessed by descending two steps from a doorway in the east gable. It is a two-storey structure with three small projecting corner towers, two of which are solid and one of which houses a spiral stair with a cap-house at the top, opening onto a narrow wall-walk. Its ground floor retains a fine ogee-headed limestone window with hood-moulding and scotia moulding on the inner jambs, a late medieval detail suggesting this tower post-dates the main church fabric. The older, poorly preserved tower at the west end survives only as a fragment: part of a north wall showing the lower springing of a barrel vault and two corbel stones, and part of a west wall containing a double-splayed loop. The scar of what may have been a connecting doorway is faintly legible on the outer face of the church's west gable. The site sits about 100 metres south-west of a motte and bailey, the raised earthwork of a Norman fortification, and close to what may have been a medieval village, suggesting this whole low ridge once formed a more densely occupied landscape than the quiet pasture field it appears today.

A graveyard still surrounds the church, and a low enclosure wall abutting the south side contains nineteenth-century graves along with a cross-inscribed stone. A blocked window embrasure in the east wall of the south-east tower has at some point been repurposed to hold a nineteenth-century headstone, a small detail that says something about how later communities made use of the spaces earlier ones left behind.