Most people walking past would take it for a field boundary, an unremarkable fold in the Cork countryside. In fact it is the last surviving fragment of what was once a large circular enclosure, the kind that typically defined an early Christian ecclesiastical settlement in Ireland.

Early medieval church sites in Ireland were commonly enclosed within a roughly circular earthen or stone boundary, known as a cashel when built of stone or a rath-like bank when built of earth. These enclosures marked the sacred boundary of a monastic or ecclesiastical community, separating the religious precinct from the surrounding landscape. The arc at Knockmacool belongs to exactly this tradition, and it sits to the north-west of Desertserges graveyard, a place whose name, derived from the Irish "diseart", meaning a hermitage or desert retreat, already signals deep ecclesiastical roots. Taken together, the surviving bank and the adjacent graveyard are the physical remnants of an early church site recorded and discussed by Hurley in 1980.

The bank itself is the thing to look for here: low, grassy, easy to underestimate, but legible once you know what you are seeing. The enclosure of which it formed part was large enough that the arc alone spans 200 metres, suggesting a settlement of some significance in its day, even if almost nothing of that settlement survives above ground.