The structure is a souterrain, an underground tunnel or chamber built during the early medieval period, typically using drystone walling and roofed with flat capstones called lintels. These features are found across Ireland, most often within or beside ringforts, and were probably used for storage, refuge, or both. What makes the example at Cloonfane quietly arresting is how much of it endures, and how clearly the boundary between ruin and survival is written in the ground itself.

The souterrain sits within a ringfort, the remains of a circular enclosed settlement of the kind that dots the Irish countryside in the thousands. On the Ordnance Survey six-inch map of 1838, the feature was already being labelled simply as "Cave", which suggests it was a visible local landmark even then, its origins perhaps already half-forgotten. At ground level today, a rectangular depression runs roughly three metres along a northeast to southwest axis, ranging from about thirty centimetres deep at the northeast end to eighty centimetres at the southwest. This sunken strip represents the collapsed and now roofless section of the original passage, its covering lintels long since displaced. Two large slabs have tilted into the depression at the deeper southwest end, the angle of their fall preserving a record of the moment the roof gave way. Just beyond them, however, a single lintel remains in place, still covering an opening that leads into a length of passage where the drystone walls stand and the roofing is intact, extending further to the southwest for several metres underground.