A souterrain, to give it its proper name, is an underground passage or chamber built during the early medieval period in Ireland, typically associated with ringforts and thought to have served as storage space, a refuge, or both. This particular example is L-shaped in plan, entered through a lintelled opening low enough to require crouching, and it descends through a passage into a chamber that sits noticeably lower than the floor of the approach tunnel, giving the whole interior an almost staged quality, as though the builder intended each transition to be felt.

The entrance opens from a sunken area near the centre of the ringfort interior. The passage runs roughly east to west for just over three and a half metres, its walls built from large, roughly rectangular stone blocks corbelled slightly inward at the top to carry the weight of massive roof lintels above. At the western end, the passage narrows to a tight constriction, just half a metre wide and sixty centimetres high, before dropping abruptly into the main chamber. That chamber is roughly oval, about three and a half metres along its north-south axis and nearly two metres wide, with a ceiling height of one point seven metres, meaning an adult can stand upright inside it. The lower walls are cut directly from bedrock, with dry-stone construction continuing above to the corbelled roof. Set into the eastern wall towards the southern end is a recessed niche that narrows toward the back, and a small, partly blocked opening behind it may be an original air vent. An earth floor covers both the passage and the chamber, with loose stones scattered near the western end of the passage and a heap of larger stones at the chamber's southern end, details that hint at disturbance or partial collapse over the centuries.