The feature exists, at least to the naked eye, only from the air, and only under the right conditions: a cropmark, where differences in soil depth or moisture cause the grass or grain above a buried structure to grow at a slightly different rate, revealing the ghost of a shape that has otherwise entirely vanished beneath the surface.

Cropmarks of this kind are one of archaeology's more quietly remarkable tools. When a ditch or bank was dug into the ground centuries ago and later filled in, the disturbed earth retains different qualities from the undisturbed soil around it. In dry summers, crops over a buried ditch stay greener longer because the looser fill holds more moisture; crops over a buried wall or bank may stress and yellow sooner. From above, these variations register as tonal differences, tracing the plan of something long gone. The circular form at Cronyhorn, approximately fifty metres in diameter, is consistent in shape with a ringfort, the most common field monument in Ireland, typically a farmstead enclosed by an earthen bank and ditch dating from the early medieval period, roughly the fifth to twelfth centuries. Whether this particular site is indeed a ringfort, or something earlier such as a Bronze Age enclosure, cannot be said without further investigation. The cropmark was identified in aerial imagery captured on 14 July 2018.