The structure is a souterrain, a type of dry-stone tunnel built during the early medieval period in Ireland, typically used for storage, refuge, or both. What makes this one quietly interesting is precisely its ambiguity: the passage exists, the flat stones exist, but the overhead structure they may once have formed is gone, or at least uncertain.

The cashel itself, a cashel being a stone-walled ringfort of the kind common across Munster, provides the context for the souterrain. Such underground passages were frequently constructed within ringforts, entered from inside the enclosure and running outward beneath the walls or toward the centre. A researcher named Hartnett, writing in 1939, recorded the passage at Dooneens in some detail, noting the large roofing stones along the tunnel and the concentration of loose stone toward its inner end. He cautiously suggested that the loose material might represent the collapsed or surviving remnants of some kind of overhead structure at the terminus. Several flat stones observed in the interior since then appear to correspond with what he described, though whether they are structural survivors or simply fallen debris remains an open question.