A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period in Ireland, and associated with settlement sites. They served various purposes, among them storage, refuge, and possibly ventilation for above-ground structures. That one exists here, in a quiet corner of Mayo, is the kind of detail that rewards the curious; a subterranean feature, invisible from the road, catalogued by name and townland but not yet described in any detail that has reached the wider world.

The townland name Cuillonaghtan derives from Irish and reflects the deep layering of place-names across Connacht, where early medieval and even older land divisions have survived in the spoken and written record long after the physical features they once described have become obscured. Souterrains in the west of Ireland are associated broadly with ringfort settlements, known in Irish as raths or cahers, and were typically constructed between roughly the sixth and twelfth centuries. Without further excavation or published survey detail for this particular site, the precise form and extent of the Cuillonaghtan souterrain remain unclear, as does its relationship to any associated surface monument.