A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, and thought to have served as a place of refuge, storage, or concealment. They are not uncommon in the Irish landscape, but they are usually at least hinted at by a subtle depression in the ground, a crop mark in dry summers, or a scatter of disturbed stone. At Lonagh, there is nothing of the sort.

The tradition places the souterrain within an associated ringfort, a type of enclosed farmstead, usually circular, built from earthen banks or stone walls, and widely used across Ireland from roughly the sixth to the twelfth centuries. The ringfort itself is recorded, but the underground structure survives only as a piece of local memory rather than as anything a surveyor has been able to confirm in the ground. Whether the souterrain was long ago collapsed and its rubble absorbed into the soil, or whether the tradition preserves knowledge of something that has simply never been investigated, remains an open question.