The cathedral dedicated to Ss Mary and Senan, he wrote, was of "no great beauty", a judgement that says more about Victorian taste than about the building itself. What stands on the eastern side of this small island in the Shannon Estuary is in fact a structure of considerable archaeological complexity, one that accumulated layers of alteration across several centuries while remaining, at its core, a single oblong cell roughly 23 metres long and just over 10 metres wide.

The lower courses of the north, south, and west walls are built from very large stones laid in irregular courses, and the west wall retains what is sometimes called cyclopean masonry, a construction style using massive, barely dressed blocks, up to the point where the gable begins. The gable itself is built of much smaller stones and leans noticeably outward. The church originated in the eleventh century, and the building's history is readable in its fabric. Two antae, projecting extensions of the side walls that are characteristic of early Irish church architecture, survive on the north and south sides, flanking a trabeate doorway, one with a flat lintel rather than an arch, that still carries mortises on the underside of its projecting inner lintel. Later medieval campaigns added pointed doorways in the north and south walls, raised the height of the building, and inserted new windows along the south wall, including a cusped ogee-headed window whose flat-arched embrasure is decorated with three carved sandstone heads. The east window, a three-light cusped ogee-head with a rosette above and a hood moulding terminating in carved animals, with a bishop's head at the apex, has lost all its tracery but remains expressive. It sits slightly off-centre, shifted a little to the north. A sacristy added to the north side was built as a structurally independent unit, its walls not bonded to the nave, and appears to have had an upper floor. About 25 metres to the west, a round tower completes the monastic cluster.

Scattery Island is accessible by seasonal ferry from Kilrush, and the site is in state care. The break between the eleventh-century nave and the later chancel is visible on the inner face of the south wall, and the step down from chancel to nave floor, nearly five metres from the east wall, is still there underfoot. The south wall, leaning outward at its eastern end and slowly separating from the east wall, gives the interior a slight sense of structural unease that no amount of conservation work has entirely resolved.