An L-shaped depression, measuring roughly six metres east to west and six and a half metres north to south, sits within the enclosure, and its geometry is too deliberate to be purely accidental. The working interpretation is that this sunken outline marks the roof-fall of a souterrain, an underground passage or chamber built from stone, typically during the early medieval period, and used for cold storage, refuge, or both. When a souterrain collapses inward over centuries, it leaves precisely this kind of telltale hollow in the earth above it.

The ringfort in which it sits is a separate recorded feature in its own right. Ringforts, roughly circular enclosures defined by an earthen bank and ditch, were the most common form of rural settlement in early medieval Ireland, and it was not unusual for their inhabitants to construct a souterrain beneath or within the enclosed area. The combination of the two here at Cartronabree, if the depression does indeed confirm the presence of an underground structure, would be a fairly typical pairing, though the collapsed state of the souterrain means its full extent and construction details remain a matter of inference rather than excavation.