That the ground level has risen so substantially around it only adds to the sense that the building has been quietly absorbed by its surroundings, half-forgotten beside the much larger cathedral nearby. What makes the structure genuinely unusual is that roof: solid stone, barrel-vaulted internally and finished with a neat outer skin, with a pointed chamber running above the nave to carry the apex. Stone roofs of this kind were deliberately fireproof, a detail that has led some scholars to propose the building served as a shrine for precious relics, though royal burial and use as a subsidiary church have also been suggested.

The oratory was built around AD 1100 under the patronage of Muirchertach Ua Briain, king of Munster and high king of Ireland between 1086 and 1119, a figure associated with several ambitious ecclesiastical projects. The nave measures roughly 8.8 metres by 5.3 metres internally, with walls of mortared yellow and brown sandstone that lean slightly inward on the north and south sides, a feature known as batter, which lends the building a subtle solidity. A chancel once extended to the east, also vaulted and roofed in stone, but this has not survived. The interior is lit only by two narrow windows, one in each side wall toward the east end, their heads formed simply from two stone slabs leaned together. The west doorway is another matter entirely: three orders deep, with short round columns, worn capitals carved with animals and foliage, roll mouldings, and faint traces of dentil ornament along the hood. A carved cross-slab lies inside the nave. Nineteenth-century drawings record the roof in a rougher state, with plant growth pushing through, before repairs brought it to something closer to its present condition.

The oratory sits approximately twenty metres north of St Flannan's Cathedral in Killaloe and is in State care as a National Monument. The building is partly below ground level, so approaching it involves descending to the oratory rather than simply walking up to it, an orientation that makes the scale and completeness of the surviving structure all the more striking once you are level with its walls.