The church has a small rectangular barrel-vaulted nave (measuring 8.78m x 5.33m internally) with an upper chamber under a steep gabled stone roof. The chancel is lost but the chancel arch, plain, with chamfered imposts, remains. Small gabled windows survive on the N and S of the nave and in the E gable of the upper chamber. There is a round-headed window at the W end of the upper chamber. The only sculptural decoration is found on the W doorway. A collection of loose carved stone including some Romanesque pieces are housed in the church.

Killaloe was the centre of Ua Briain power in the late 11th and 12thc., and was the burial place of Toirrdelbaig Ua Briain(1086) and Muirchertach Ua Briain (1119). It was established as a see at the synod of Rathbreasail in 1111 (see Killaloe Cathedral).

R. Gem (forthcoming) has recently argued that the oratory was probably built under the patronage of Muirchertach O'Briain around 1100, and that the W doorway was the work of an Anglo-Norman mason. The design of the doorway, with recessed colonnettes, volute and animal capitals, and a moulded arch, has parallels in Anglo-Norman sculpture of the late 11thc. The decoration shows no evidence of any of the Romanesque features such as chevron and cushion capitals which became common in Irish Romanesque afterc.1120. The oratory thus shows an awareness of Anglo-Norman Romanesque in Ireland before the introduction of the more decorative early 12thc. Romanesque forms introduced in the 1120's at Cashel and Lismore. The loose fragments 1, 2a and b, 3 and 4 were not part of the oratory and their original provenance and setting is unknown. Westropp (1893, 195,197) illustrated fragments 2a and b, 3, and 4, together with another chevron fragment similar to 3 but with a palmette in the triangle, as being at Clarisford; Keane (1867, 370) records that some sculptures were found in the river Shannon and moved to the bishop's demesne at Clarisford. The sculpture is evidently by the same workshop as the s. doorway of the cathedral, and probably dates from c.1200. It is also closely related to the loose sculpture at Tuamgraney; the jamb stones 2a and b are identical in design to two stones recently found in the partition wall at Tuamgraney (see Tuamgraney, VI, iv a and b). The gable finial in the cathedral (VI.v) probably comes from St Flannan's oratory. See also bibliography for Killaloe Cathedral.