A souterrain, which is an underground stone-lined tunnel or chamber typically associated with early medieval settlements in Ireland, was once recorded within the interior of a cashel here. A cashel is a dry-stone ringfort, a circular enclosure built to protect a farmstead and its inhabitants, usually during the early medieval period. The souterrain would have served as a place of refuge, storage, or both, accessed from inside the enclosure. Today, no surface trace of it survives.

The record of its existence rests on a single source: McCaffrey, writing in 1952, who noted the souterrain in the interior of the cashel at Mannin, cataloguing it as entry number 110 in a broader survey of the region. That record preserved something the landscape itself no longer shows. Whether the souterrain collapsed, was backfilled, or simply became buried under subsequent centuries of soil accumulation is not known. What remains is the cashel itself and the knowledge that something once ran beneath it.