On a north-east-facing slope at Glenacroghery in County Cork, the ground shows nothing, no bank, no ditch, no trace of any kind that something was once deliberately made here. And yet the cartographers who produced the first Ordnance Survey six-inch map of Ireland in 1842 recorded a circular enclosure roughly twelve metres in diameter, its western edge running up against a north-south field fence. Since then, whatever boundary or feature they were able to observe has been absorbed entirely into the landscape.

The structure, when it existed above ground, would have been what archaeologists broadly class as an enclosure, a category that covers everything from early medieval ringforts, which were farmsteads enclosed by an earthen bank and ditch, to prehistoric ceremonial sites or animal pounds. At twelve metres across, this was a modest feature, smaller than a typical ringfort but not impossibly so. The 1842 Ordnance Survey mapping, carried out with considerable care by the Royal Engineers and local surveyors, often captured earthworks and enclosures that local people could still identify at the time, meaning that something visible on the ground was being recorded, not merely inferred. Whether the enclosure was already fading by then, or was subsequently levelled by agricultural activity, the field fence that once abutted it to the west is now the only spatial clue that anything ever stood at that particular point on the slope.