It is a souterrain, a type of man-made underground structure associated with early medieval Irish settlement, typically used for cold storage, refuge, or both. This one is modest in scale but precise in its construction: a single chamber measuring roughly 3.7 metres long, 1.1 metres wide, and 1.4 metres high, rectangular in plan but curved upward in cross-section, giving it a barrel-like profile when viewed from inside.

The structure was described in detail by Twohig in 1973, who noted that the walls are built in roughly coursed stonework, with the upper courses corbelled inward, a technique where each stone projects slightly beyond the one below it until the gap is narrow enough to be bridged by flat roofing slabs. It is a method that requires no mortar and no arch, just careful placement and the weight of the stones themselves working in compression. At the western end, there is a blocked opening that may originally have served as a ventilation shaft or as the primary entrance to the chamber. Its blocking suggests either deliberate closure at some point or later structural collapse, though the record does not specify which.