It is a souterrain, a type of man-made subterranean passage or chamber built in early medieval Ireland, typically associated with storage, refuge, or both, and this one sits quietly within an ecclesiastical enclosure, close to the enclosing wall in its south-western quadrant.

The entrance is narrow by any standard, just over half a metre wide and less than half a metre high, opening into the western end of a stone-built chamber roughly four metres long. Inside, the space widens slightly towards the west, giving the chamber a trapezoidal plan. The walls are drystone, built with care from small and medium-sized stones, and the roof is formed from massive overlapping slabs carried on a course of corbels, projecting stones set at the top of the side walls to help bear the weight above. At the north-eastern corner, at floor level, there is a low lintelled opening that has since been blocked up. This may have been a creep, the term used for a low connecting passage between one souterrain chamber and the next, suggesting the structure once extended further. About four metres to the north-east of the main entrance, a shallow hollow in the ground surface points to the likelihood that it did, with subsidence hinting at a continuation of the underground passage in that direction. The full extent of the souterrain, then, remains only partially known.