A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, used for storage, refuge, or both. This one has collapsed, and what remains visible at ground level amounts to little more than a handful of small hollows and surface depressions near the south-western bank of the enclosure, the kind of subtle disturbance in the earth that most people would walk past without a second thought.

The rath itself, a roughly circular earthen enclosure of the early medieval period, is the dominant feature of the site. Such enclosures were the farmsteads of their era, and it was not uncommon for their inhabitants to construct souterrains beneath or within the banks. At Cabragh, local knowledge rather than any excavation or formal investigation has preserved awareness of the souterrain's existence. Unusually, the interior of the rath also contains a house, suggesting the enclosure has remained in some form of practical use long after its original purpose had passed.

The evidence for the souterrain is largely negative, defined by absence and subsidence rather than anything a visitor could clearly identify. Those few hollows near the south-western bank are the only surface clue that something once lay beneath, and their modest scale is a fair measure of how thoroughly the structure has been reclaimed by the ground.