Somewhere in the farmland of this part of County Galway, a cashel once stood, a type of early medieval ringfort defined not by earthen banks but by a substantial drystone wall enclosing a roughly circular space. This one was considerable in scale, recorded by McCaffrey in 1952 as having a diameter of approximately 47.2 metres. Within or close to its walls there was also a souterrain, an underground stone-lined passage of the kind commonly associated with cashels and ringforts, used variously for storage or refuge. Both the cashel and the souterrain were significant enough to be marked on the third edition of the Ordnance Survey map in 1933. Today, neither leaves any trace on the surface.

The cashel at Ballymanagh was most likely destroyed in the early 1970s when the area was bulldozed, according to information attributed to Professor Rynne. The timing is not unusual. The decades following Irish land reclamation schemes saw the clearance of a great many ancient monuments across the west of Ireland, as farmers levelled field boundaries, mounds, and stone structures that had survived for over a millennium. A drystone wall of nearly fifty metres across, sitting in the middle of workable farmland, would have presented an obvious obstacle. McCaffrey's 1952 classification preserved the record of what had been there, but the physical structure was gone within a generation of that documentation.