The site contains a souterrain, an underground stone-built tunnel or chamber of the kind commonly associated with early medieval settlement in Ireland, most likely positioned in the south-western quadrant of the enclosure. Its entrance has been sealed off, not by archaeologists or preservationists, but for a far more practical reason: the opening was considered a hazard to grazing livestock.

The souterrain lies within a rath, the circular earthen enclosure familiar across the Irish countryside, typically dating from the early medieval period between roughly the sixth and twelfth centuries. Souterrains were built as annexes to these settlements, used variously for storage, refuge, or concealment. They tend to run for several metres underground, sometimes connecting multiple chambers, and their entrances were usually low and narrow enough to slow any intruder. At Ballygowan, local knowledge preserves the awareness that this one exists at all, even if the entrance itself is now closed. That detail is itself telling: the community remembered the passage long after it ceased to be safely accessible, and the decision to block it was a practical accommodation between an ancient structure and the ongoing business of farming the land above it.