A small ruined church, located north of the round tower, cathedral and oratory. It consists of a nave and a chancel 7.3 m x 5.1 m and 3.3 m x 3.27 m respectively (Westropp, 1900); with some remains of a Romanesque chancel arch. The nave walls and E gable are complete, the chancel walls approx. 1.6 m high. The round-headed E window has a wide interior splay and a rebuilt arch, the S window of the nave has a damaged opening and repaired interior. The S door, with a pointed arch, is later medieval.

The monastery was founded by St Senan (d.544?), probably in the 6thc. The first recorded abbot d. 727, and coarbs of Senan are recorded down to 1081. The island was plundered by Norsemen in 816, 835 and possibly occupied c.974-7, and recaptured by Brian Borumna. It was plundered by the Danes of Dublin in 1057 and in 1101 when the monastery was destroyed. It was not chosen as a see at the synod of Rathbreasail in 1111, but appears to have been established as a bishopric soon after; O Lennain, erenagh or coarb of Inis Cathaig, was also bishop when he died in 1119. The bishopric of Inis Cathaig is listed as a suffragen of Cashel in 1152 on Cardinal Paparo's list. Normans from Limerick sacked the monastery in 1176, and the churches were desecrated by William Hoel, an Englishman, in 1178. In 1188 Aed O Beccan died as bishop of Inis Cathaig. The monastery appears to have been incorporated into the diocese of Killaloe around this time (Gwynn and Hadcock, 1970, 96-97). The ruins were repaired by the Board of Works in 1880-81.

The E window with a plain round-headed splayed opening appears to be 12thc. although it has a later medieval segmental rear arch. Westropp (1900) describes the chancel arch with 'clustered pillars now fallen'; the inaccurately replaced jambstones are of two different types and appear to come from two orders, or from a doorway and chancel arch. They are of the same design as those in the oratory on the island.