Others exist only as a faint discolouration in a field, legible solely from the air at the right moment of a dry summer. The circular enclosure at Carrigacunna in north County Cork belongs to the second category. Its presence was recorded not by excavation or ground survey but by a single aerial photograph taken in July 1970, in which a cropmark traces the line of a fosse, a defensive or boundary ditch, forming a near-complete circle roughly thirty metres across.

Cropmarks appear when buried features such as ditches or walls affect the growth of surface vegetation. A filled-in ditch retains more moisture than the surrounding soil, so the crops above it grow taller and greener; a buried wall does the opposite. From ground level these differences are invisible, but from altitude, particularly in dry conditions when the contrast is sharpest, the underlying archaeology surfaces as patterns of colour and tone. The Carrigacunna photograph captured exactly this: a ghostly ring indicating where a fosse once enclosed a circular area, most likely the boundary of a ringfort or similar early medieval settlement enclosure, a type of monument extremely common across Ireland but often surviving only in this fragmentary, subterranean form. The dimensions, around thirty metres in diameter, sit at the smaller end of the ringfort scale, suggesting a single farmstead rather than a site of particular status.