A hollow roughly a metre long, a metre wide, and just over half a metre deep interrupts the northeast quadrant of the enclosure, opening onto what appears to be an earth-cut souterrain, the kind of underground tunnel or chamber that early medieval communities in Ireland constructed beneath or beside their farmsteads. Whether used for food storage, refuge, or some combination of both, souterrains are found throughout Ireland, but this one is inaccessible, its passage sealed or collapsed beyond the point where the hollow opens. In the southeast quadrant of the same ringfort, two flat stones lie exposed at ground level, possibly connected to the same underground structure.

Ringforts, which are circular enclosures defined by one or more earthen banks and ditches, were the dominant form of rural settlement in early medieval Ireland, in use roughly between the fifth and twelfth centuries. It was common practice to incorporate a souterrain within them, and the association here is consistent with that pattern. The ringfort at Cúil An Bhuacaigh carries its own monument number and sits as a distinct record from the souterrain within it, suggesting both features have been documented and considered separately. A preservation order under the National Monuments Acts protects the site, meaning it cannot be interfered with or altered without legal sanction.