The only evidence of its existence comes from the air, where the buried feature betrays itself as a crop mark, a faint circular discolouration in growing vegetation caused by the differential moisture and nutrients retained in the disturbed earth of a long-filled ditch. It is the kind of site that could be overlooked indefinitely without an aerial photograph taken at precisely the right moment.

Ring-ditches of this type are generally understood to be the remains of prehistoric funerary or ceremonial monuments, often the ploughed-down remnants of round barrows or burial mounds whose earthen banks have long since been flattened by centuries of agricultural activity. What endures is the circular trench that once defined the monument's boundary, preserved only as a ghostly outline in the subsoil. The Knocknagull example is known from aerial photographic evidence alone, making it one of many such sites across Ireland whose existence depends entirely on the chance conditions, dry summers, particular crop types, the angle of light, that allow the landscape to briefly reveal what lies beneath it.