It is a souterrain, an underground stone-lined tunnel of the kind built throughout early medieval Ireland, typically for storage, refuge, or ventilation of a settlement. The entrance opening, roughly a metre across in either direction, sits in the western half of the cashel interior, about five metres from the enclosing wall. Below that opening, the passage runs on an east-west axis, its walls built in drystone technique and its ceiling formed from massive roofing slabs.

A cashel is a roughly circular enclosure defined by a drystone wall, and this one contains the souterrain as a deliberate feature of its original design rather than a later addition. The passage is partly infilled now with loose stones and soil, so its full extent cannot easily be read from above, but something of its path can still be traced at ground level. A slight linear rise, a low scarp about a metre wide and roughly twenty-five centimetres high on its northern side, runs across the cashel floor from the opening at the west all the way to the cashel wall at the east-south-east. It is the buried passage pushing back against the surface, its presence made legible by the faintest topographic shadow.