A souterrain is an underground or semi-underground passage or chamber built from dry stone, typically dating from the early medieval period and associated with nearby settlement or enclosure. Here, only a single subrectangular chamber survives, roughly three metres in length, its roof formed by the flat capstones that were laid across the walls when it was built. Much of the chamber has since collapsed, leaving only a fragment of what was once a deliberate, carefully constructed underground space.

The cashel within which it sits, a cashel being a stone-walled ringfort of the early medieval period, provides its immediate context. The souterrain occupies the centre of this enclosure, which is the position most likely to have kept it sheltered and concealed, perhaps used for storage or as a place of refuge. The pairing of cashel and souterrain is not unusual across Ireland, but finding one still partially intact within its original enclosure gives Castlecarra a particular coherence. The surrounding landscape, close to Lough Carra in south County Mayo, is one where early medieval activity has left scattered traces, and this site sits quietly among them.