A souterrain is a man-made underground passage or chamber, typically associated with a nearby settlement and used variously for storage, refuge, or ventilation of a dwelling. Most are built from dry-stone walling, which makes this one somewhat distinctive: the vertical sides of the chamber appear to have been cut from the living rock itself rather than built up with laid stone.

The chamber runs to just over five metres in length and barely a metre wide, sloping downward to the south from its entrance. The roof is formed by large flat lintels, the horizontal slabs laid across the top of the chamber, with packing-stones wedged in to stabilise them. At the southern end, debris appears to have fallen in through a small secondary opening, suggesting the structure was once more extensive, or that another feature connected to it from below. The position of the souterrain on a south-east-facing outcrop close to the northern shore of the inlet is deliberate rather than incidental. Directly across the water on the opposite shore sits a rath, a circular enclosed settlement of the early medieval period typically defined by one or more earthen banks and ditches. The spatial relationship between the two sites, souterrain on one shore, rath on the other, raises the obvious question of whether they were part of the same community, with the inlet serving as a boundary or a convenience rather than a barrier between them.