That uncertainty is, in its own quiet way, the most interesting thing about the site. A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval raths, the circular enclosed farmsteads that once dotted the Irish countryside in their thousands. They were used variously for storage, refuge, or ventilation, and their presence beneath a rath is not unusual. What is unusual here is the ambiguity: local knowledge insists one exists, but the ground itself has not yet confirmed the claim.

The rath at Ballybeg contains two depressions that have attracted attention. The first is an elongated hollow, between one and a half and three metres wide and up to seventy centimetres deep, sitting on a raised area in the north-western quadrant of the interior. It curves from the south-west, where it is shallower and narrower, towards the north-east, where it runs up against a townland boundary wall that cuts across the rath interior. That curving, narrowing profile is suggestive of a collapsed souterrain passage, the kind of subtle surface sag that appears when an underground structure finally gives way. The second depression is a shallower, roughly circular hollow roughly five metres across, sitting against the inner edge of the rath bank in the north-eastern quadrant. Neither feature, however, shows any visible stonework or structural detail that would settle the matter one way or the other. The townland wall bisecting the rath interior adds its own layer of complexity, a later boundary cutting through an earlier landscape and obscuring whatever might lie beneath.