That sunken floor is unusual enough, but what makes this site genuinely arresting is what lies beneath it. Cut into the southern face of the upper terrace, behind a low lintelled opening barely 80 centimetres high, is a souterrain, an underground passage built from dry-laid stone, of a kind used throughout early medieval Ireland for storage, refuge, or both.

The souterrain here is a careful piece of construction. Its slightly corbelled walls, where the stones are stepped inward as they rise to narrow the gap before a roof of flat slabs closes it off, carry a basal row of intermittent upright stones and boulders along their base, a detail that suggests considered building rather than hasty digging. The passage does not run straight. From the entrance it extends northward for about 2.7 metres, then curves east for more than four metres, then turns north again for another 3.5 metres. Near the northeastern angle a creep way branches off, though it is no longer passable, and beyond it a further passage, now infilled, continues east for at least two metres. The total underground distance curves and bends in a way that would have made the interior difficult to navigate quickly and impossible to observe from any single vantage point, qualities that suit a place intended for concealment. The hut above measures just over four metres in internal diameter and was built in drystone, that is, without mortar, using the same technique as the underground structure below it. The archaeological survey of the Iveragh Peninsula, compiled by A. O'Sullivan and J. Sheehan and published by Cork University Press in 1996, recorded both the surface structure and the underground passage as a single complex site.

The upper terrace on which the hut stands is some 20 metres long and raised about 1.2 metres above the terrace below it, giving the whole arrangement a layered, almost deliberately obscured quality. Anyone approaching from the lower ground would have seen a modest rise of earth before any structure became visible, and the entrance to the souterrain, tucked into the terrace face, would not have been obvious at all.