A slightly raised strip of earth, roughly two metres wide and scattered with loose stones and several flagstones, runs from the centre of the enclosure out towards its northeastern entrance. To a casual eye it might read as collapse or field clearance, but the line is too deliberate, too directed, to be either.

What lies beneath is a souterrain, an underground passage or chamber built from drystone walling and roofed with large flat slabs, a type of structure associated throughout Ireland with early medieval settlement, typically dated to the first millennium AD. Souterrains were used variously for storage, refuge, or both, and are frequently found within cashels, the circular stone-walled enclosures that served as farmsteads or minor strongholds during the early Christian period. The cashel here at Caherbarnagh, recorded separately, provides the wider context: the souterrain was an integral working feature of whatever community once occupied this high Cork ground, not an isolated curiosity but a component of a larger, organised site. The flagstones now visible at the surface likely represent the collapsed or exposed roofing of the passage, the weight of centuries gradually pressing the structure back into the hillside.