To most passers-by it would read as a dip in the grass, a soft anomaly in an otherwise unremarkable patch of land. To an archaeologist, it marks the likely collapse point of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, typically associated with nearby settlement and used for storage, refuge, or both.

The souterrain sits within the northern half of a ringfort, the circular earthwork enclosure that was the standard unit of rural settlement in Ireland from roughly the sixth to the twelfth century. Ringforts varied considerably in their construction and elaboration, but the combination of a defended enclosure with a concealed underground chamber below it was not unusual. The souterrain would have offered cool, stable conditions for preserving food, and in times of threat, a place to hide or to exit unseen. At Ballymacowen, the ground itself has subsided slightly over the centuries where the roof of the underground structure has given way, leaving that telltale hollow as the only surface evidence of what lies beneath.