This is a souterrain, an underground passage or chamber built from drystone masonry, typically associated with Early Medieval settlement in Ireland. Their exact purposes remain debated, though storage and refuge are the most commonly cited uses. What makes this one quietly compelling is its setting: not beneath a farmhouse or field, but tucked into the fosse of the rath, that is, the defensive ditch between the inner and outer enclosing banks of the fort itself.

The passage enters from its south-western end, runs straight for a stretch, and then curves to the east, terminating at a squared wall with what appears to be a creep at its base. A creep is a low, deliberately narrow opening between chambers, the kind that would slow or stop an intruder while allowing a person who knew it to pass through. The passage measures more than three metres in surviving length, between roughly half a metre and just over a metre in height, and under a metre wide at its broadest point. A stretch of drystone walling extending a further 2.3 metres to the south-west of the entrance suggests the original passage was longer, with the south-western portion now lost or collapsed. The associated rath, recorded separately, places this structure within a wider complex of Early Medieval activity at the site.