This is what archaeologists call a cropmark, a ghost of a structure revealed when buried ditches or banks, which hold moisture differently from undisturbed ground, cause the vegetation above them to ripen at a slightly different rate. Seen from the air on a dry summer, that variation in colour can sketch out the outline of something that might otherwise go unnoticed for centuries more.

The enclosure at Gortrush was first identified from an aerial photograph taken on 22 July 2000, and the circular form remained faintly legible on later satellite imagery as well. Circular enclosures of this kind are among the most common archaeological features in the Irish landscape. They range from prehistoric ring ditches and Bronze Age burial monuments to the ringforts, known in Irish as ráth or lios, that were the farmsteads of early medieval Ireland, typically dating from roughly the fifth to the twelfth centuries. Without excavation, it is not possible to say which of these categories the Gortrush enclosure belongs to, and the fifty-metre diameter alone does not settle the question. What is harder to explain away, and perhaps more arresting than the archaeology itself, is that an ESB electricity pole now stands roughly in the centre of the enclosure, planted there long after whatever the original structure was had already slipped below the surface of the land.