A souterrain, the term for an artificial underground passage or chamber typically associated with early medieval settlement in Ireland, once ran beneath this spot. It is now closed up, its precise extent unknown, and what remains visible is less a structure than an absence: a depression in the earth tracing a line south-east from the ringfort towards the Clashavoon stream.

The feature was first formally noted by Coleman in 1947, who described what he observed as a trench that looks like a collapsed souterrain. That phrasing is telling. Even then, the structure had already lost its defining form, leaving only a linear hollow that suggested rather than confirmed what lay beneath. The ringfort itself, a type of enclosed farmstead common across early medieval Ireland, provides the wider context; souterrains are frequently found within or adjacent to such enclosures, where they may have served as storage spaces, refuges, or both. Local knowledge has kept the association with the Clashavoon stream depression alive, though the underground passage itself remains inaccessible and unexcavated.