A ruined church that quietly preserves the evidence of its own alterations is relatively rare. This one in Palmerstown, Co. Kildare, manages to hold several centuries of architectural revision in a single partially restored shell, including a large two-centred arch rebuilt off-centre in the west gable wall, which still incorporates the tufa-built jamb of an earlier, wider arch. That earlier opening was not demolished so much as absorbed; its southern jamb remains visible in the surrounding masonry, a ghost of a different building embedded in the one that replaced it.

The church is thought to have originated with the Knights Hospitallers of St John of Jerusalem, a medieval military and religious order whose ownership of the land gave the nearby village of Johnstown its name. The rectangular structure, measuring roughly 13.4 metres east to west internally, is built of rubble limestone with occasional tufa and granite boulders, and may represent what was once a chancel, the eastern section of a church reserved for the clergy, with the nave now entirely lost. The east gable retains a restored double ogee-headed window, its sill steeply angled into a broad square-headed embrasure. Elsewhere, a round-headed window-piece has been repurposed as a facing for a small interior niche, functioning as a stoup, the shallow basin near a church entrance used for holy water. The interior holds three items of particular note: a fifteenth-century monument associated with the Flatesbury family, a medieval font, and a nineteenth-century high cross erected over the burials of the Bourke family of Mayo, three objects spanning four centuries placed within a few metres of one another.