A souterrain, which is an underground stone-lined passage or chamber typically built during the early medieval period, lies in the south-western quadrant of a ringfort in this townland, and it leaves no visible trace whatsoever on the surface above it. No dip in the ground, no hollow, no tell-tale scatter of stone. It is simply there, recorded and catalogued, folded into the earth.

Souterrains are found across Ireland in considerable numbers, almost always in association with ringforts, the circular enclosed settlements that were the dominant form of rural habitation from roughly the fifth to the twelfth centuries. Their exact function is still debated, though most archaeologists consider them to have served as places of refuge, cool storage for foodstuffs, or both. The one at Lackaduv sits within or beside a ringfort, and beyond that association, the record is quiet. No excavation appears to have been carried out, no dimensions are known, and the structure's condition underground remains undocumented.