A souterrain is an underground stone-built passage or series of chambers, constructed without mortar, typically associated with early medieval ringforts and used for storage, refuge, or both. What makes this one particularly arresting is not just its scale but its deliberate complexity: the changes in direction between chambers suggest a structure designed to disorient, slow down, or conceal.

Access into the souterrain now comes through a surface collapse close to the south-south-west bank of the rath interior, an opening roughly two metres across that drops into a narrow creepway only 0.56 metres wide and 0.56 metres high. That passage opens into the first chamber, which runs approximately 5.3 metres on a north-west to south-east axis, rising to about 1.3 metres in height. Its walls are built from large stones and boulders in rough courses, with the uppermost two courses slightly corbelled inward to bear a roof of massive lintel slabs. A low stone-edged platform at the north-west end fronts the lintelled opening into the next creepway, which curves northward before emerging into a second chamber, this one oriented north to south and measuring roughly five metres in length. At floor level in the rounded north-east corner of that chamber, a third, tighter opening leads through a short passage into a third chamber, noticeably narrower and lower than the others, its ceiling pressing down to around one metre at the western end. The third chamber extends eastward for 1.7 metres before an impassable blockage of collapsed earth cuts off further progress, leaving only a gap of 30 to 40 centimetres between the debris and the roof lintels. The chamber clearly continues beyond that point, its full extent still unknown.