What lies beneath the surface, however, is a souterrain, an underground stone-built passage of the kind constructed in early medieval Ireland, typically for storage, refuge, or both. This one runs to more than nine metres in length and is laid out in a roughly L-shaped plan, its long axis oriented north-west to south-east, with a shorter arm branching off at the south-east end toward the north-east and south-west.

The construction method is drystone, meaning the walls were assembled from carefully fitted stones without the use of mortar, a technique that required considerable skill to execute at depth and under ground pressure. Souterrains of this type are found across Ireland and are generally associated with early Christian period settlement, often positioned close to a ringfort or similar enclosure, though no such associated feature is mentioned here. What makes the Ballinduff example quietly notable is its setting. The turlough visible from the slope is itself an unusual feature of the Irish midlands and west, a lake that fills and empties seasonally through the limestone bedrock rather than through surface drainage, giving the surrounding landscape a rhythm that shifts markedly between winter and summer. A souterrain placed to overlook such a feature, whether by intention or coincidence, sits within a landscape that has always been subject to disappearance and reappearance.