The collapse of earth has left an opening roughly a metre wide, and what lies beyond it is a curving underground passage seventeen metres long, narrow enough that a person must stoop, its walls cut directly from the earth and finished above with courses of dry-laid stone to carry the heavy roof lintels overhead.

This is a souterrain, a type of man-made underground structure found widely across early medieval Ireland, typically associated with ringfort settlements and thought to have served for storage, refuge, or both. The Carrigrour example follows a path that runs first northward before turning a short distance to the west, giving it the curved profile that was sometimes deliberately built into these structures, possibly to obstruct a direct line of sight or movement from the entrance. The walls are mostly earth-cut rather than fully stone-lined, though the drystone courses near the roof suggest careful construction where it mattered most, where the weight of the lintels needed support. Notably, a second souterrain lies roughly 110 metres to the north-northeast, raising the possibility that this hillock once formed part of a more substantial early settlement whose surface features have long since disappeared into the pasture.