The uncertainty is part of the record. A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, used variously for storage, refuge, or ventilation. This particular one exists, for now, only as a pencil mark on an old map.

The mark appears on the first-edition Ordnance Survey six-inch map, the great nineteenth-century cartographic undertaking that documented Ireland in extraordinary detail. Someone, at some point, added this annotation by hand, placing the possible souterrain roughly 150 metres east of Trinity Church in Brockagh. That church is a known monument in its own right, and the proximity suggests the two features may once have formed part of the same early ecclesiastical landscape. Whether the pencilled location reflects a local tradition, a visible feature since lost, or something glimpsed and never properly recorded, is no longer possible to say. The ground itself has closed over the question. The area indicated on the map is now entirely covered in dense furze, the low thorny scrub that colonises abandoned or disturbed ground across the Irish uplands, making any close inspection of the surface impossible.