No mound, no ditch, no stone, no visible trace. Yet an earthwork is recorded here, catalogued by archaeologists and assigned its own national monument reference. Its existence rests entirely on a moment of light and shadow caught from the air in the summer of 1986.

The site was identified through an aerial photographic survey centred on the Bruff area of County Limerick, when a circular cropmark appeared in the photographs, catalogued under reference numbers 126.01 and 126.02 from the survey, with the corresponding aerial photograph logged as AP 5/2069. Cropmarks form when buried features, such as the filled-in ditches of ancient enclosures, affect how crops or grasses grow above them; a circular ditch, even one long since ploughed flat, may still hold moisture differently from the surrounding soil, producing a ring of greener or more vigorous growth that becomes legible from altitude in the right conditions. This particular mark suggested a circular enclosure, a form found widely across Ireland and associated with a broad range of periods and purposes, from prehistoric settlement to early medieval farming. A second enclosure was noted lying roughly 30 metres to the west. Significantly, neither feature appears on Ordnance Survey Ireland's historic mapping, which means they were not recognised at ground level during any of the nineteenth or twentieth-century surveys that shaped the standard record of Irish monuments.

By the time satellite imagery became available, the cropmark had apparently ceased to register. Neither the Digital Globe orthoimage captured between 2011 and 2013, nor subsequent Google Earth imagery, shows any surface remains or visible trace of the feature. The site lies in pasture, 110 metres south-east of the Morningstar River, which here marks the boundary between Ballygrennan and the neighbouring townland. A visitor standing in that field today would see only grass. The record was compiled by Fiona Rooney and uploaded to the national monuments database in July 2021, meaning the site's formal existence as a known archaeological feature is, in a sense, quite recent. What actually lies beneath the soil, its date, its function, its relationship to the adjacent enclosure, remains entirely open.