Others exist as little more than a shadow in a field. The ring-ditch at Ballylegan in County Cork belongs firmly to the second category. It came to light not through excavation or chance discovery, but through an aerial photograph taken in July 1989, when the cropmark of a circular fosse, roughly ten metres in diameter, showed up against the surrounding ground. A fosse is simply a ditch, and when one lies buried beneath a field, the soil above it retains moisture differently from the soil around it, causing the crops growing there to ripen at a slightly different rate. From ground level, nothing is visible. From the air, in the right season, the outline appears.

Ring-ditches of this kind are found across Ireland and are generally understood to be the eroded or buried remnants of much earlier monuments, often Bronze Age burial mounds whose central mound has long since been ploughed or weathered away, leaving only the circular ditch that once surrounded it. The Ballylegan example is notably small, at around ten metres across, and the aerial photograph suggested a possible entrance on the south-eastern side, though without excavation that detail remains tentative. Its existence was formally recorded in the Archaeological Inventory of County Cork, Volume 4, covering North Cork, published in 2000.