In 1995, a souterrain, apparently blocked, was noted close to the enclosing bank of the earthwork, its entrance sealed and its interior unknown.

A rath is a roughly circular enclosure defined by an earthen bank and ditch, typically dating from the early medieval period in Ireland and associated with farmsteads or defended settlements. Souterrains, the stone-lined or rock-cut underground passages often found within them, were used variously for storage, refuge, or concealment. The blocking of a souterrain entrance is not unusual; some were sealed deliberately in antiquity, others became choked with debris over time. What survives at Bellasallagh is described only as a possibility, the ground retaining just enough of a trace to be noticed and recorded, without yielding anything further.