What survives is a single intact section, roughly a metre and a half long and the same across, sitting somewhere near the middle of what was once a tunnel stretching over eleven metres from end to end. The rest has fallen in on itself, and the only clue to the full extent of the original structure is a depression running across the southern half of the enclosure, with faint traces of side-walls visible at its edges.

A souterrain is an underground passage or chamber, built without mortar using a technique known as drystone construction, in which carefully selected stones are stacked so that their weight and fit hold them in place without any binding material. They are found across early medieval Ireland, typically inside or close to a rath, the circular earthwork enclosure that formed the basis of a farming settlement during the first millennium. Their exact purposes are still debated, though storage, refuge, and ventilation for living quarters have all been suggested. The rath at Cahernamona, recorded separately as GA097-047, provides the context for this souterrain, which runs on a north-south alignment within the enclosure's western portion. In the northern half, collapsed roof lintels, the flat stones that would have spanned the passage overhead, are still visible lying where they fell, giving a clear sense of how the structure was originally roofed and how thoroughly time has undone it.