The only evidence that anything is there at all is a cropmark, the kind of faint discolouration that appears in aerial photography when buried features affect how grass or crops grow above them. A ditch filled with looser, moister soil will encourage slightly lusher growth overhead, while a buried wall or compacted surface does the opposite. The result, invisible to anyone walking the field, can become legible from the air under the right conditions of light, season, and drought.

The circular feature at Johnstown measures approximately 22 metres in diameter and is defined by what appears to be a single encircling ditch. It was noted from a Google Earth orthoimage captured on 16 March 2021, though a fainter outline had already appeared in Digital Globe aerial photography taken sometime between 2011 and 2013. Circular earthworks of this kind are common across the Irish midlands; many are the remains of ringforts, which were enclosed farmsteads typically dating from the early medieval period, roughly the sixth to the twelfth century. Others turn out to be Bronze Age burial enclosures or the remnants of later features. Without excavation, the Johnstown example cannot be assigned to any particular period or function with confidence. What the aerial evidence does confirm is that the ditch is real, that it encloses a roughly circular area, and that it has been sitting quietly beneath the grass while the land above it was drained and improved into the kind of tidy agricultural field that betrays nothing of what lies underneath.