Roughly rectangular, about eleven and a half metres long and nearly four metres wide at its broadest point, the hollow sits within the western sector of a rath, the type of enclosed farmstead built in their thousands across Ireland during the early medieval period. It is not much to look at from the surface, but the shape and dimensions are consistent with a souterrain, an underground stone-lined passage or chamber used for storage, refuge, or both, common features of raths that are often only detected by the telltale sinking of the ground above them.

Souterrains were typically constructed by corbelling or lintelling stone over a dug trench, then backfilling the earth on top, which means that as the structure deteriorates over centuries, the surface gradually settles and slumps. That process appears to have left its mark here. The parent rath at Carra, recorded separately, would once have been a substantial enclosed settlement, and the presence of a possible souterrain within it fits a well-established pattern of how such sites were organised and used during the early medieval centuries in the west of Ireland.