Others exist only as a faint discolouration in a field, visible for a matter of weeks each summer when drought stresses the grass unevenly above buried features. The enclosure at Ballydague in north Cork belongs to this second, quieter category. What is known of it comes from a single aerial photograph taken in July 1989, which captured a cropmark outlining the fosse, or ditch, of a circular enclosure roughly 30 metres in diameter. At ground level, there is almost certainly nothing to see.

Cropmarks form when buried ditches or pits retain moisture differently from the surrounding undisturbed soil. In dry conditions, crops or grass growing above a filled-in fosse stay greener for longer, or conversely, ripen faster, depending on the season and the depth of the feature. The result, from the air, is a ghostly ring or arc that maps the outline of something long since levelled. The Ballydague cropmark traces a circular enclosure of modest size, comparable in scale to the smaller class of ringfort, the most common field monument type in Ireland, typically a farmstead enclosed by one or more banks and ditches during the early medieval period, roughly the fifth to twelfth centuries. Whether this particular site dates from that period is not established from the available evidence; the cropmark alone tells us the shape and approximate dimensions, not the date or function.

Because the site survives only as a subsurface feature detected through aerial photography, there is no upstanding monument to visit, and no particular vantage point that would reveal anything to a person standing in the field. The interest lies less in the place itself than in what the method of its discovery suggests: that the landscape of north Cork, like much of rural Ireland, almost certainly contains many more such features, waiting for the right summer, the right drought, and a camera overhead at the right moment.