A roughly circular enclosure, somewhere between forty and forty-five metres in diameter, shows up as a cropmark, the kind of ghostly outline that appears in aerial photography when buried features affect how grass or grain grows above them. The feature in question is the cropmark of a fosse, a defensive ditch, whose presence was first caught on an aerial photograph taken on 13 July 1989.

Cropmark archaeology of this kind depends on a simple principle: buried ditches tend to retain moisture and nutrients, so the vegetation above them often grows taller or stays greener longer than the surrounding crop, tracing the outline of the original cut in ways no ground-level survey would easily detect. The Annamult enclosure belongs to a broad class of circular earthworks found across Ireland, many of them associated with early medieval settlement, though without excavation it is not possible to say more about its date or function. What is clear is that the enclosure remained detectable not only in the 1989 photograph but also, faintly, in later satellite imagery. A modern field boundary meanders roughly northwest to southeast just outside the northeastern sector of the circle, the kind of coincidence that sometimes suggests farmers working the land over many generations retained an instinct for where the ground behaved differently, even when the original earthwork had long since been ploughed flat.