This one in Ballygrennan, County Limerick, exists almost entirely as a photograph, a single aerial image that captured something the ground itself no longer seems willing to admit. What appear to be the outlines of an ancient field system, a network of boundaries that once divided and organised this landscape, are visible only as linear cropmarks: subtle variations in the colour and growth of vegetation above ground that betray buried features beneath. Walk the field today and you would find nothing out of the ordinary.

Cropmarks form when buried walls, ditches, or other sub-surface features affect how crops or grass grow above them. Soil disturbed by old ditches tends to retain more moisture, producing lusher growth, while buried stonework does the opposite, stressing the vegetation above it. These differences, invisible at ground level, can become legible from the air, particularly in dry conditions. The Ballygrennan site, recorded as Site No. 039141, was identified through exactly this process during examination of aerial photographs taken on 3 November 1984. Those photographs were collected not by archaeologists but by Bórd Gáis Éireann during survey work for the Curraleigh West to Limerick gas pipeline, one of those practical infrastructure projects that, almost incidentally, generated a remarkable record of the Irish landscape from above. The site sits in reclaimed pasture just east of the townland boundaries with Ballinlee and Ballinlee North, with a separate enclosure recorded approximately 100 metres to the north. Neither feature appears on Ordnance Survey Ireland historic maps, suggesting they were already invisible at the time those surveys were carried out.

By the time orthophotography was taken of the area between 2005 and 2012, no surface traces of the field system were visible, and a Google Earth image from September 2020 confirmed the same. The site is, in practical terms, not something a visitor could locate or observe without access to the 1984 aerial photograph itself. What makes Ballygrennan worth knowing about is precisely that absence; it is a place whose archaeological significance exists in a single archival image, compiled into the national record by Fiona Rooney and uploaded in April 2021, preserved in a database rather than in the soil. The surrounding countryside near the Ballinlee townland boundaries is ordinary-looking farming land, and that, in a way, is the whole point.