A souterrain is an underground stone-built tunnel or chamber, typically constructed during the early medieval period and associated with nearby settlement enclosures. They were likely used for storage, refuge, or both, and their deliberate complexity, with turns and constrictions, may have made pursuit difficult for anyone unwanted. This one at Ballynamanagh is largely collapsed and in poor condition, but its underlying geometry remains legible.

The structure sits within the south-western quadrant of a rath, which is a circular earthen enclosure of the kind used by farming families across early medieval Ireland. The souterrain was built using drystone construction, meaning the walls were assembled without mortar, and its ground plan bends through four identifiable sections. Starting nearest to the enclosing bank and running roughly south-south-west to north-north-east for about six metres, the passage then turns and heads north-west to south-east for four metres, before turning again to run north-east to south-west for seven metres. The final section, running east to west, ends in a collapsed pile of stones roughly four and a half metres long, among which a single exposed lintel stone is still visible. The reference to this site by McCaffrey in 1952 suggests it was recorded during a period of broader regional survey work in the mid-twentieth century, though the structural ruin was already evident by that point.