A souterrain, to use the proper term, is an artificially constructed underground passage or chamber associated with early medieval settlement in Ireland, typically drystone-built and roofed with flat lintels. What makes the one at Barnycarroll particularly interesting is its plan: rather than a single corridor, it forks, sending one arm curving on a roughly northwest to southeast axis through the interior of the rath, and a second branch peeling off to the southwest until it meets the inner face of the enclosing bank.

The construction detail that survives in the intact sections is considerable. The main passage, around seven metres long and less than a metre wide, has side walls of three to four courses of large stones and boulders, with slight corbelling near the top to carry the roof lintels. The floor is earthen. Midway along, a raised bank of earth and stone crosses the full width of the passage, beyond which the headroom drops sharply to around 0.46 metres, meaning anyone moving through it would have been forced to crawl. This kind of deliberate constriction is a recognised feature of souterrains, thought to slow or obstruct anyone attempting a forced entry. More unusual here is a two-tiered creepway, a stacked pair of lintelled openings set one directly above the other in the same wall, the lower barely 0.46 metres high and the upper only marginally larger. These give onto the branching passages. The southwestern arm leads through a small, roughly D-shaped chamber, partly silted up with soil, and then continues to the bank, where collapse near the surface has left a lintel exposed and another is visible protruding from the bank's inner face. The southeastern end of the main chamber has already collapsed into a grassed-over depression roughly a metre deep, though the northwestern end remains intact, its drystone walls still standing 1.5 metres high under two in situ lintels.