The structure is a souterrain, an underground gallery of the kind built throughout early medieval Ireland, typically as a place of refuge, storage, or concealment, constructed by setting upright stone slabs along the sides of a trench and covering them with flat capstones or lintels. Here, one of those lintels has given way, and what lies beyond the entrance remains effectively inaccessible.

The passage appears to run northward from its opening in the modified bank. The bank itself has clearly been altered at some point, and the souterrain's entrance is cut into its outer, westward-facing side, suggesting a relationship between the two features that has not been fully resolved. The Iveragh Peninsula, where Ballycarbery sits, is dense with early medieval remains, and souterrains like this one are frequently found in association with ringforts, the circular enclosed farmsteads that were the dominant settlement form of the period. Whether that is the case here is not stated, but the modified bank raises the question naturally enough.