The structure is oblong, with an unusual rounded northwestern side, and its walls still stand to nearly two metres. What draws the eye, however, is not the building itself but what lies underneath: a subterranean passage, drystone-built, running northward from below the structure for at least two metres before continuing a further four metres beyond it. The passage is roughly half a metre high and just over a metre wide, its roof now mostly gone, though one collapsed roofing slab remains in place, a rectangular stone of about a metre across with a small perforation set slightly off-centre. A second passage, partially filled in and visible only as a line of flat surface slabs, lies about two and a quarter metres to the southwest. Nobody has established with any confidence what either passage was for.

The valley's Irish name, Fothair na Manach, meaning something close to "the terrace or woodland of the monks", hints at an early ecclesiastical presence, as does a persistent tradition linking the site to St. Brendan, the sixth-century navigator celebrated in the Navigatio Sancti Brendani. Brendan is said to have founded a monastery here and, in another version of the tradition, to have rested at Fohernamanagh before setting out on his legendary ocean voyage. Neither claim can be verified archaeologically, but the valley's sheer remoteness is itself considered suggestive of monastic settlement, since early Irish monks often sought out deliberately difficult terrain. What complicates any straightforward reading of the site is that, despite everything, the valley is recorded as having been home to three or four families around the beginning of the nineteenth century, and it is not clear how much of what survives today reflects that comparatively recent habitation rather than any earlier monastic phase. The building with its subterranean passages sits on a levelled platform retained by a short stretch of low drystone walling, itself interrupted by a small drystone recess whose lintelled roof has partially collapsed; even the platform's construction was deliberate and considered, which only deepens the uncertainty about the whole ensemble's original purpose.