What betrayed it was not stone or earthwork but the behaviour of a cereal crop in summer heat, the plants above its buried ditch collapsing at a slightly different rate to those around them, tracing out a ring on the ground that no one standing in the field could have read.

The site at Killaclug was recorded during aerial survey in August 1984, when a photograph captured the cropmark of a fosse, the term for a ditch, typically dug as part of a defensive or boundary enclosure, forming a circle roughly 35 metres in diameter. Cropmarks of this kind appear when buried features alter the soil's capacity to hold moisture: a filled-in ditch retains more water than the surrounding subsoil, producing lusher or, in drought conditions, more stressed vegetation above it. The enclosure itself belongs to a familiar class of circular earthwork found throughout Ireland, most commonly associated with the early medieval period, though without excavation its date remains unknown. It exists, for now, as a single aerial image and a measurement.