A souterrain, an underground passage or chamber typically built during the early medieval period, often of dry-stone construction, and used variously for storage, refuge, or concealment, is recorded at this site, though it announces itself only through its own failure. The ground simply gave way.

The collapse is thought to have occurred in the south-western quadrant of a ringfort, the circular earthwork enclosure within which the souterrain sits. Ringforts were the most common form of rural settlement in early medieval Ireland, and they frequently contained souterrains as ancillary structures. At Carrigoon, local information attributes the subsidence to ploughing, suggesting that agricultural activity broke through whatever thin layer of earth or stone was holding the roof of a buried chamber in place. No visible surface trace now remains, meaning the depression or void that once betrayed the structure has since been obscured, filled in, or simply grassed over. What is recorded here is less a monument than the memory of one, preserved in an account of the ground giving way beneath a plough.