The ring-ditch, a circular ditched enclosure roughly ten metres across, exists only as a cropmark, a faint signature readable from the air but entirely invisible at ground level. Cropmarks appear when buried features such as ditches or pits affect the soil's moisture retention, causing the crops or grass above them to grow at slightly different rates or to a slightly different colour. From altitude, that variation resolves into a shape. From the ground, it is just a field.

Ring-ditches of this kind are generally associated with prehistoric funerary or ritual activity, often representing the encircling ditch of a burial mound whose earthen material has long since been ploughed flat. They turn up across Ireland and Britain with some regularity, their survival owed less to any robustness of construction than to the fact that the ditch cutting persists in the subsoil long after everything above ground has vanished. The example at Coolcull Big is notably small, at approximately ten metres in diameter, and its presence was recorded from aerial photography rather than any excavation or field survey. What lies beneath, whether a cremation, an inhumation, a cenotaph, or something else entirely, remains unknown.