It is a souterrain, a type of man-made subterranean chamber or tunnel constructed during the early medieval period, typically between the seventh and twelfth centuries. Built by hand from dry-stone walling and covered with large capstones, souterrains were dug beneath or beside ringforts and settlement sites across Ireland, serving variously as places of refuge, cold storage, or concealment. That one exists at Ballinlassa is itself a quiet indicator that this corner of Mayo was once a settled, organised community, the kind of place where someone had the labour, the knowledge, and the reason to build something intended to endure underground.

Beyond the fact of its existence and location, the details of this particular souterrain remain sparse in what is publicly available. No dimensions, no record of excavation, no named landowner or discoverer has yet been made accessible. What can be said is that Mayo's landscape holds a considerable number of such monuments, often associated with the raths and cashels of early Christian Ireland, and that souterrains in the west of the country are frequently cut into glacial subsoils or constructed within earthen banks, their presence betrayed above ground only by a subtle depression or a scatter of displaced stone.