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.

The three re-used capitals may come from the adjacent oratory, where they may have formed part of the chancel arch. They appear to have been angle capitals, with human heads filling the angle, a type common in Irish Romanesque sculpture (compare examples at Annaghdown (Galway), Inchagoill (Galway), Dysert O'Dea (Clare), St Saviour's Glendalough (Wicklow), Killeshin (Laois), Timahoe (Laois), Kilmore (Cavan), Kilteel (Kildare), Rahan (Offaly), Duleek (Meath) and Inisfallen(Kerry)). Another similar head capital or corbel has been removed from Scattery Island to the Scattery Island Centre (see under Kilrush (Clare)).