A souterrain is an underground stone-built passage or series of chambers, typically associated with early medieval settlement in Ireland, and thought to have served variously as cold storage, refuge, or concealment. What distinguishes this one is not merely its survival, but the ingenuity of its internal design, which seems engineered to slow or obstruct anyone moving through it without permission.

The structure was uncovered during excavation of the associated house, and described by Hayden in 2000. Entry began at a circular shaft, partially blocked by a fallen stone, which opened into a low creepway just 0.4 metres high. A vertically set slab partially obstructed the passage into the first chamber, which had unlined earthen sides and partially collapsed roof lintels. Between the first and second chambers, a so-called porthole slab forced passage through a gap only 0.3 metres above floor level, with a notch cut into the underside of the slab to allow a body to squeeze through. The second chamber narrowed to as little as 0.4 metres in width before leading onward. The third chamber, the largest at 4.6 metres long, was lined on most sides with rough drystone-walling and included a small blind outshoot, a dead-end alcove, on its western side. A fourth creepway, just 0.4 metres wide and 0.45 metres high, connected to a fourth chamber, where a possible air-vent was observed at the western end. Crucially, the excavator established that the souterrain predated the house built above it: several of its roof lintels actually underlay and partly supported the walls of the hut. Chambers three and four extend beyond the footprint of the house entirely, running outward beneath the surrounding ground.