A souterrain, to use the correct term, is a man-made underground structure, typically stone-lined, built during the early medieval period and associated with ringforts across Ireland. They served various purposes: refuge, storage, ventilation for living quarters above. The one at Clonmoyle is, by all accounts, invisible. There is no surface trace, no telltale depression in the grass, no obvious sign that anything lies beneath.

The earliest modern record of it comes from P. J. Hartnett, writing in 1939, who noted what he called a 'cave' located roughly eight yards south of the north-east entrance, positioned inside the main rampart of the ringfort. Even then, it was described as 'closed up' and 'barely noticeable'. That was over eighty years ago. The ringfort itself, a roughly circular enclosed settlement of the kind built throughout early medieval Ireland, survives as a separate recorded monument. The souterrain within it has, for now, simply disappeared back into the ground that made it.