On 16 July 1971, an aerial photograph captured what is known as a cropmark, the faint but legible trace of a buried fosse, or defensive ditch, showing through the differential growth of crops above it. The outline it revealed was roughly square, measuring around 35 metres on its northeast to southwest axis, and it was not alone.

Immediately to the southeast, the same photograph picked out a second enclosure, slightly larger and also sub-square in shape, which appears to be conjoined with the first at their shared corner. Cropmark evidence of this kind is read by archaeologists as the ghost of a former enclosure, a type of enclosed settlement that was common across early medieval Ireland, typically defined by one or more earthen banks and ditches and used for farmsteads, religious sites, or other organised habitation. The two enclosures at Dunbell Big seem to have been physically linked, suggesting they may have functioned together as part of a single complex rather than as separate, unrelated sites. What makes the location more striking still is the density of similar sites nearby. Several other enclosures have been identified within a radius of roughly 330 metres, clustering in a pattern that points to sustained and organised activity in this landscape over a long period.