What lies beneath, or what once lay fully beneath, is a souterrain, an artificially constructed underground passage or chamber system built without mortar, used in early medieval Ireland typically for cold storage, refuge, or both. This one has been partly swallowed by time and by the hands of whoever later needed the stone more than the structure, but enough survives to make its original shape legible.

The plan is roughly U-shaped: two parallel east-west passages set about two metres apart, joined at their western end by a small rectangular chamber. The southern passage, around 5.5 metres long, is now roofless, its lintels robbed out, though sections of side walling survive. The northern passage is longer, somewhere between nine and ten metres, and its western third remains roofed and intact. Connecting the two passages is a narrow creep, a low, deliberately constricted opening that would have forced anyone moving through it to crouch or crawl, a design feature common to souterrains and thought to slow down or deter intruders. The chamber walls are built from short, thin, neatly coursed rectangular slabs of local stone, corbelled gently inward from mid-height to carry the roof lintels. Those lintels are relatively thin, and several in the northern passage are already cracked or slightly displaced, which likely explains why so much of the roof has failed. In the south wall of the northern passage there is a small stone-lined recess, just 40 centimetres wide and 25 centimetres high, extending 70 centimetres back into the wall before meeting an earthen face. Its purpose is unrecorded. To the west of the souterrain, a grassed-over rectangular hollow with stones protruding from its edges may or may not be related to the structure; the relationship is not clear. Low, sod-covered scarps running across the field to the west and north appear to be later field boundaries that post-date the souterrain entirely, a reminder that the landscape here has been divided and redivided long after whoever built this underground system had gone.