No parallel for it has been identified at any other early Irish church, and whether it belongs to the original build or was added later remains an open question. It sits above a Romanesque doorway of four orders, itself a replacement for an earlier three-order door put in during restoration work in 1879, which was itself a replacement for whatever first stood there. Layers of intervention, in other words, are built into the very fabric of the place.

The church sits at the centre of the monastic complex on Inis Cealtra, an island long associated with early Christian settlement in the west of Ireland. Its oldest surviving fabric is most probably 10th century, visible in the more westerly window of the south wall, which has a flat lintel, inclined sides, and an interior sill built up in steps, all features consistent with pre-Romanesque construction. The original building was a single-celled nave with corner antae, projecting stones at either end of the side walls that are a characteristic feature of early Irish ecclesiastical architecture. A Romanesque chancel was later inserted between the east antae, sometimes referred to separately as St Colum's church; it is neatly built in ashlar masonry but is not bonded to the nave, making the join between the two periods clearly legible to anyone who looks carefully. A chancel arch with three decorative orders facing the nave carries a grotesque head on its keystone. The restored altar inside is a dressed stone block with bowtell mouldings at the angles, a type of roll moulding, capped with floral capitals; scholarly opinion holds that monumental altars of this kind are exceptionally rare, with only two other convincing examples known, both also in County Clare, at Dysert O'Dea and Rath. A small square aumbry, a wall recess used to store liturgical vessels, survives on the south wall near the chancel arch. The Board of Works carried out extensive restoration in 1879 to 1880, and the Office of Public Works reroofed the nave in the 1990s.

The interior now shelters a substantial collection of early crosses and cross-slabs, propped along both the north and south walls. Two graveyards adjoin the church, one to the south and the 'saint's graveyard' immediately to the east, while a round tower stands just eleven metres to the southwest. Excavations in 1974 uncovered burials west of the church, adding yet another period of use to a site whose occupation spans well over a millennium.