The site is recorded as a souterrain, an underground stone-lined tunnel or chamber typically built during the early medieval period, often associated with ringforts and thought to have served for storage, refuge, or both. The catch is that there is no visible surface trace of it whatsoever. Its existence rests on local knowledge rather than physical evidence anyone can point to.

The souterrain is believed to lie within a ringfort, one of the thousands of roughly circular enclosed settlements that dot the Irish countryside, most of them dating from roughly the sixth to the tenth centuries. Ringforts and their associated souterrains were once so common across the landscape that even where the earthworks have been levelled by centuries of farming, memory of them can persist in place names, field boundaries, and the recollections of people who worked the land. At Mullagh, that oral tradition is apparently the only thread connecting the present to whatever lies beneath.