A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with nearby settlements or raths; they were used variously for storage, refuge, or as escape routes. At Dunmoran, the knowledge of one has survived only as local tradition, unconfirmed by excavation or precise survey.

The rath itself, a type of circular earthwork enclosure used as a farmstead in early medieval Ireland, sits at the heart of this quiet puzzle. The souterrain's existence has never been formally verified, and its location within or around the ringfort remains unestablished. That a memory of it persists at all is itself telling. Local oral tradition has preserved knowledge of archaeological features long before, and sometimes long after, any official record takes notice, and the fact that this particular tradition was recorded suggests it was considered credible enough to warrant attention, even without physical confirmation.