A souterrain, as such passages are known, is a stone-lined underground tunnel or chamber typically associated with early medieval settlement, used variously for storage, refuge, or purposes still debated by archaeologists. This one, in the valley known as Fohernamanagh or Fothair na Manach, is unusual not only for its setting but for the layers of uncertainty that surround it. The building above measures roughly 4.4 by 2.5 metres internally, with drystone walls reaching up to 2 metres in height. The passage beneath runs about 4 metres to the north-northwest, standing only around half a metre high and 1.2 metres wide. One collapsed roofing slab remains inside, roughly a metre square, with a small perforation slightly off-centre. What the perforation was for is not recorded. A second apparent passage lies about 2.25 metres to the southwest, now completely filled in, visible only as a line of flat slabs on the surface.

The valley itself adds another dimension of strangeness. Fothair na Manach, meaning roughly the hollow of the monks, is described as the most inaccessible settlement on the mainland of Corca Dhuibhne, the westernmost peninsula of Kerry. Tradition associates it with St. Brendan, who is said to have founded a monastery here and to have spent time in the valley before setting out on the famous sea voyage attributed to him. Despite this isolation, the valley was reportedly home to three or four families at the start of the nineteenth century, according to Power writing in 1923. Whether the structures visible today belong to an early monastic phase, to that later domestic occupation, or to some combination of both, remains genuinely unclear. The function of the building and its relationship to the souterrain beneath it has not been resolved.