The opening is barely enough to admit a person: eighty-six centimetres wide and only twenty-five centimetres high, a slot cut into a raised area northeast of where the centre of a rath once stood. A rath is a roughly circular earthen enclosure, typically dating from the early medieval period, used as a farmstead and occasionally as a place of refuge. This one has been levelled, its interior much disturbed, but the souterrain it once sheltered has proved harder to erase.

Souterrains are dry-stone underground passages or chambers, built without mortar, that appear across Ireland in association with raths and other early medieval settlements. Their exact purpose is still debated; they may have served for storage, for hiding people or valuables, or as escape routes. The Ballindrehid example follows a familiar pattern: a lintelled entrance, meaning the roof is formed from flat stones laid horizontally across the walls, leading into a passage roughly a metre wide that runs to the west-northwest for several metres before the collapsed earth blocks any further progress. At the point of blockage, what appears to be a creepway opens toward the southwest. Creepways are narrow constrictions within a souterrain, thought to slow or deter intruders, and their presence often signals that a more complex system of passages lies beyond, whether accessible or not.

The site is visible from ground level only as a partial excavation in the raised area, with the lintelled opening exposed. The rath that once defined this space has left little obvious trace above ground, making the souterrain entrance the sole legible remnant of an enclosure that was, at some point, deliberately cleared away.