A souterrain is an underground stone-built passage or series of chambers, typically associated with early medieval ringforts and enclosures, and thought to have served as storage, refuge, or both. What makes this one quietly peculiar is that three separate openings pierce the cashel wall, a cashel being a stone-built circular enclosure of the early medieval period, and the passages and chambers behind them suggest a structure that may run deeper into the site than anyone has properly mapped.

The souterrain's walls are drystone-built, meaning the stones are laid without mortar, and the roofs are formed from flat slabs laid horizontally across the top. The western opening, now collapsed, leads into a small chamber no more than two metres at its widest, with a lintelled gap cut into its southern end. To the north-east, a second opening leads into a passage barely half a metre high and less than a metre wide, a tight crawl by any measure. A third opening, a short distance south of that, accesses another small chamber standing roughly eighty centimetres high. A report from the early 1940s raised the possibility that the souterrain extends further, reaching beneath the interior of the cashel itself, though that remains unverified.