Others exist only as a shadow in the soil, visible for a few weeks a year when crops grow unevenly over buried features below. The enclosure at Ballynaboul in North Cork belongs to this second, quieter category. What is known of it comes from a single aerial photograph taken in July 1989, in which a curved arc of discolouration traces a path through a field, running roughly from north-northwest to south-southeast. The arc is the surface expression of a fosse, a defensive or boundary ditch, long since filled in and forgotten at ground level.

Cropmarks of this kind form when buried ditches retain more moisture than the surrounding soil, causing whatever is planted above them to grow taller or greener, producing a pattern legible only from the air and only under the right conditions of drought and growth. The arc at Ballynaboul suggests the remains of a roughly circular or oval enclosure approximately thirty metres in diameter. Enclosures of this general type are among the most common archaeological monument forms in Ireland, often associated with early medieval settlement, though without excavation it is impossible to assign a date or function to this particular example with any confidence. The thirty-metre estimate places it at the smaller end of the scale, closer to a ringfort used for domestic purposes than to a larger ceremonial or hilltop enclosure, but that remains speculative.