It is easy to walk past without registering it as anything more than a slight irregularity in the field. The enclosure measures roughly 26 metres along its north-west to south-east axis and about 19 metres across, making it a modest but clearly deliberate feature in the landscape. What draws the eye, once you know to look, is that it does not stand alone: a second enclosure of the same general type lies roughly 94 metres to the south-south-east.

Enclosures of this kind, defined by earthen banks rather than stone, are a recurring feature of the Irish countryside. They are broadly associated with early medieval settlement, typically dating from somewhere between the fifth and twelfth centuries, though without excavation it is rarely possible to assign a precise date or function to any individual example. Some enclosed a farmstead and its immediate yard; others may have served as livestock pens or had ritual significance. The pairing of two such features in relatively close proximity at Castlefergus is itself of interest, since grouped enclosures sometimes reflect the organisation of a small farming community, with separate spaces for dwelling and animals, or perhaps successive phases of occupation on the same ground.