A circular enclosure, defined by a fosse (a defensive ditch dug into the earth), lies buried beneath the soil, invisible to anyone walking across it. Its existence is known only because of what crops do when their roots meet disturbed or differently compacted earth: they grow at slightly different rates, producing faint variations in colour and height that, from the air, resolve into clear geometric shapes. This phenomenon, known as a cropmark, has revealed countless prehistoric and early medieval sites across Ireland that would otherwise remain entirely unknown.

The enclosure at Tinnaranny came to light through an aerial photograph, reference GB89.AN.20, which captured the cropmark of the circular form with its surrounding fosse still legible in the landscape after what may be many centuries of burial. Circular enclosures of this kind are among the most common archaeological features identified through aerial survey in Ireland, and they span a wide range of periods and functions, from prehistoric ritual enclosures to the ringforts of the early medieval period, which served as farmsteads for a single family and their livestock. Without excavation, it is rarely possible to say with certainty which category a given cropmark belongs to, and Tinnaranny is no exception. What the photograph preserves is the outline, the geometry, the fact of the thing.