Situated c. 24 m E of the round tower, the cathedral has a simple rectangular plan measuring c. 20.80 m x 8.40 m, incorporating the side walls and W end and part of the E wall of an earlier church (Westropp, 1897). The antae and trabeated W doorway with inclined jambs survive from this early church; the upper walls and E gable were rebuilt in the later medieval period. There are later medieval doors in the N and S walls near the W end of the nave, and three Gothic windows in the S wall. A later sacristy (8.10 m x 3.00 m) is attached to the N wall.
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.
Set into the wall above the central window are three re-used angle capitals with human heads.
L capital: badly damaged, but similar to central and R capitals.
Central capital: set diagonally into the wall, with a human head on the angle, with clearly defined eyes, indented pupils and a beard.
R capital: as central capital, without indented pupils, but with more clearly defined beard and curling hair.
A. Gwynn and R. N. Hadcock, Medieval Religious Houses: Ireland, London, 1970, 96–7.
D. F. Gleeson, 'The Collegiate Church of Iniscathaigh', North Munster Archaeological Journal, 2 (1940–41), 14–30.
M. Keane, Towers and Temples of Ancient Ireland, Dublin, 1867, 377.
T. J. Westropp, 'Scattery Island and Canon's Island, Co. Clare', Journal of the Royal Society of Antiquaries of Ireland, 27 (1897), 276–86.
T. J. Westropp, 'The Churches of County Clare, and the origin of the ecclesiastical divisions in that county', Proceedings of the Royal Irish Academy 22 (1900), 169–70.