A souterrain, to use the term for these early medieval subterranean structures, typically served as a place of cold storage, refuge, or concealment, built by hand from carefully arranged dry stone without any mortar. At Ballysheen, the system was uncovered by the landowner at some point in recent decades and subsequently closed up again, which means the site now exists in a peculiar state of documented absence, known to have been there, confirmed underfoot, but inaccessible.

The clearest account of what lies below comes from an inspection carried out in 1969 by John Deady, whose findings were published in the Journal of the Kerry Archaeological and Historical Society in 1972. At that time, four chambers were accessible, aligned along a roughly west-south-west to east-north-east axis. Each chamber was beehive-shaped, meaning the walls curved inward as they rose, corbelled upward in the manner common to drystone construction in early medieval Ireland, and the whole structure was built without mortar, relying entirely on the weight and placement of the stone. To gain entry, the capstone of one chamber had to be removed. Deady recorded co-ordinates and a map reference for the site, but these turn out to be incorrect, which introduces a small but nagging uncertainty: it is not entirely clear whether the souterrain he inspected in 1969 is the same one the landowner later uncovered and resealed, or whether there may be more than one such structure in the area.