The Ordnance Survey recorded it by that name on the six-inch maps of both 1838 and 1931, which suggests the label was already well established in local speech long before either survey team arrived.

The structure is a souterrain, an underground chamber or passage built during the early medieval period, typically associated with a rath, the ringfort-style enclosed farmstead that surrounds it here. Souterrains were used for storage, refuge, or both, and were constructed without mortar, relying instead on careful stonework to hold their shape. At Ardacarha, the chamber is roughly two metres wide and sits just over half a metre below the interior ground surface of the rath. Its walls are built from large, rough irregular stones that corbel inward, meaning each course projects slightly over the one below to support heavy stone roof lintels rather than a true arch. A semicircular section of the rath's outer scarp has been quarried away, exposing a vertical face through which the entrance, standing 1.3 metres high, is visible. The floor slopes downward from west to east. The quarrying that opened it up is undated, and it is unclear whether it was deliberate investigation or simply the result of agricultural clearance.

The chamber is largely obscured by brambles and difficult to examine closely, which perhaps explains why it has attracted little attention despite its clear structural detail and its long paper trail on the historic maps.