A souterrain, to give it its proper name, is an artificial underground passage or series of chambers built in early medieval Ireland, typically of drystone construction and roofed with large flat stones. This one sits within Parkmore ringfort, known in Irish as Lios Páirc Mhóir, and its visible remains amount to little more than a stone lintel, roughly 0.95 metres wide, resting on drystone walling and half-swallowed by turf. The chamber or passage behind it shows only as a gentle rise in the ground, about 4.5 metres long and 0.3 metres high. Nothing about it suggests what lies, or once lay, beneath.

In 1851 a writer named Cooke described the souterrain in enough detail to follow its full plan. There were three chambers connected by low, narrow creeps, the term for the crawl-through passages linking one section to another. The first gallery ran south-westward from the entrance and was a generous space by underground standards, twenty-six feet long and six feet high, its walls of rough-set large stones and its ceiling of immense flags. From there, a passage barely two feet wide forced anyone moving through it onto their hands and knees, until it terminated abruptly in a wall. The only way forward was upward through a square hole in the ceiling, just one foot nine inches across, which opened into a small chamber seven feet long and four feet high. To continue, a person had to drop back down through another such hole and crawl a second passage into a final gallery, this one running at right angles to the first, fourteen feet long and nine and a half feet wide. A further passage from that gallery led outward as a kind of sally-port, breaking through the inner rampart of the fort to the exterior. Concealed flags, flush with the floor of the middle chamber, covered the vertical openings between levels, so that anyone unfamiliar with the arrangement would pass over them without noticing. As Cooke observed, the design meant that from whichever end an occupant might be pursued, they retained the means to resist or escape. The ringfort itself was already marked on Ordnance Survey maps from 1842; the souterrain did not appear on those maps until the 1915 edition, where it was labelled simply as a cave.