A subcircular enclosure, roughly 30 metres across on its north-to-south axis, shows up in aerial photography not as any surviving earthwork but as a cropmark, the kind of ghostly discolouration in a field's vegetation that betrays buried ditches beneath. Where a fosse, the defensive or boundary ditch surrounding an enclosure, was once dug and later filled, the soil retains different moisture levels from the surrounding ground, and in the right conditions those differences show in the crops above.

The photograph that revealed it was taken in July 1989 as part of the Cork Aerial Survey and Aerial Photography programme. What it captured is a subcircular outline whose northern side runs unusually straight, suggesting a deliberate flattening of what is otherwise a curved perimeter. There is also a trace of a second outer fosse along that northern edge, hinting that whoever used this place saw some reason to reinforce or elaborate the boundary at that particular point. A separate circular enclosure sits approximately 45 metres to the south-west, and the proximity of the two raises quiet questions about whether they were contemporary, related, or simply sharing the same stretch of workable land across different periods. Enclosures of this general type in Ireland are frequently associated with early medieval settlement, the kind of ringfort farmstead that was the basic unit of rural life between roughly the fifth and twelfth centuries, though without excavation the date and function of the Knockacappul example remain open.