Measuring roughly eight metres in length and two and a half metres wide, aligned on a northeast to southwest axis, it sits in the northern part of the enclosure's interior and is thought to be the remains of a souterrain, an underground passage or chamber typically built during the early medieval period, often used for storage, refuge, or ventilation. What makes Ballybaun quietly unusual is that this is not the only such feature on the site. A second souterrain has been recorded in the southern sector of the same enclosure, suggesting that whoever once occupied this rath invested considerable effort in what lay below ground, not just above it.

The rath itself, a roughly circular earthen enclosure of the kind that once numbered in the tens of thousands across Ireland, forms the wider context for both features. Raths, sometimes called ring forts, were the farmsteads of early medieval Ireland, typically dating from around the sixth to the twelfth century. The presence of two souterrains within a single rath is not unheard of, but it is far from standard, and the Ballybaun example raises questions about the scale or complexity of whatever settlement once occupied the site. The northern hollow, cautiously described as a possible souterrain rather than a confirmed one, retains enough of its original form to be visible as a depression in the ground, even if much of its structure has long since collapsed or been obscured.