Souterrains, man-made underground passages or chambers typically associated with early medieval ringforts, were built for a range of purposes including storage and refuge. This one is unusual in that it was carved not from stone in the conventional sense but directly out of boulder clay, the dense glacial deposit that underlies much of the Irish countryside, making the act of cutting it as much a feat of digging as of construction.

The souterrain came to light in 1984, located in the south-western quadrant of the ringfort it belongs to. It consists of a single chamber, roughly 4.5 metres along its north-south axis and no wider than 1.3 metres at its broadest point, with a height of around one metre. That last dimension is worth pausing on: a person inside would have been unable to stand upright. At the north-western end there is a construction shaft, now partially collapsed, which would have allowed access during the original building work. The southern end features dry-stone walling, the careful stacking of unmortared stones, which served either as a revetment to hold back the surrounding earth or as a support for the roof overhead. Following its investigation, the chamber was backfilled, returning the ground above it to something close to its former appearance.