A souterrain is an underground stone-built passage or chamber, typically associated with early medieval ringforts and thought to have served as storage space, a refuge, or both. That the cartographers of 1837 and again in 1919 noted this one at all suggests it was a visible feature of the landscape for a long time before any formal record was made of it.

The rath itself, a type of circular enclosure defined by an earthen bank and ditch, forms the immediate context. Within it, close to the southern scarp, there is a cavity roughly a metre across at the top and 1.2 metres deep. At around 0.8 metres below the surface, on the northern side of this cavity, a stone-built passage becomes visible. Its entrance is roofed with a stone lintel resting on side walls of large boulders, the construction straightforward and solid. The passage runs northward through the interior of the rath. About five metres further on, an area of surface subsidence, partly the work of animal burrows, may trace the passage's continuing line underground. In one of those burrows, a large stone slab is visible, possibly another lintel, hinting at how far the structure extends without fully revealing itself.