It exists, for now, only as a cropmark, a faint circular stain roughly 32 metres in diameter that showed up in aerial photography in the summer of 2018.

Cropmarks form when buried features alter the soil above them. A filled-in ditch, for instance, retains more moisture than the surrounding ground, causing the crops or grass above it to grow slightly taller or stay greener longer, while a buried wall has the opposite effect. From the air, particularly during dry spells when these contrasts are most pronounced, the outlines of long-vanished structures become legible in the vegetation. The Kilmullin enclosure was picked out from Google Earth imagery captured on 24 June 2018, a date that places it squarely in the kind of dry early summer conditions that tend to bring these features to the surface, so to speak. Circular enclosures of this general scale are commonly associated in Ireland with early medieval ringforts, which were enclosed farmsteads typically surrounded by one or more earthen banks and ditches, though without any excavation it would be premature to assign this particular site to any period or function.