A nave and chancel church (9.10 m x 5.10 m on the exterior and 3.90 m x 6.60 m internally). The N and S wall and southern return of the E wall stand almost to full height. The remainder of the E wall and NE corner have collapsed. The SE quoin shaft and corbel, a plain S window and the remains of an E window indicate an early 13thc. date for the chancel. The nave is a later medieval addition, but incorporates numerous worked stones from an earlier structure in its fabric. The church stands within a modern graveyard with a second church, dedicated to St Connell, and a number of early Christian inscribed slabs. The site is located on an island that can be reached by foot at low tide.

The history of the site is obscure. The Annals of the Four Masters record the ‘demolition’ of Inishkeel in 618AD by Failbhe Flann Fidhbhadh, suggesting that a monastery may previously have been established at the site. The architecture of the two churches suggests building activity on the island site at least until the 15thc. Pococke, writing in 1752, observed that the Catholics buried their dead around one church, and the Protestants around the other, but makes no mention of them still being in use at that time.

The moulded fragments preserved in the walls of St Mary’s church suggest the presence of an unusually sophisticated late 12th or early 13thc. building in this remote location. Waterman (1964) highlighted the rarity of the double-keeled motif, which in a west of Ireland context is used in the W window at Boyle Abbey. The motif is also found at Christchurch, Dublin in the Gothic nave. The moulding profile with the single keel on the arris is found on the east windows at Inishmaine (Mayo), Killaloe cathedral (Clare) and Ballintober (Mayo), like Boyle, all buildings linked to the so-called School of the West. The moulded jamb sections have less obvious parallels, although the plinths recently uncovered on the Romanesque portal at Killaloe may originally have supported jambs of a very similar form, which would also have been surmounted by, albeit more elaborate, scallop capitals. With the exception of the Killaloe portal (c. 1200) practically all of these parallels can be placed c. 1220. A direct parallel for the moulding of the E window can be found in the nearby church of Termon, while looser comparisons may be drawn with the fragments at the site of the Cistercian monastery at Assaroe, a daughter house of Boyle. It is tempting to see the introduction of the Transitional style to southern Donegal as a direct result of works at Assaroe.