Beneath the graveyard at Ardkill, something older than the graves themselves is quietly visible. A slightly raised oval of ground, measuring roughly thirty metres east to west and twenty metres north to south, sits to the south-west of the site's centre, its edges defined by a low scarp between half a metre and just under one and a half metres high. That modest earthwork is all that remains of what may be an early ecclesiastical enclosure, the kind of curvilinear boundary that early Irish monasteries and church settlements used to demarcate sacred ground from the world outside.

These oval or circular enclosures are a recurring feature of early medieval religious life in Ireland, typically established between the sixth and ninth centuries. The rounded boundary, rather than a rectilinear one, is generally considered a marker of early foundation, predating the more formal geometries that arrived with later medieval church-building. At Ardkill, the sequence is legible even now: the enclosure likely came first, and a medieval church was subsequently built within the northern sector of that earlier boundary, as was common when a site retained its religious significance across several centuries. The church itself is a separate monument, and its presence within the older earthwork suggests the place held continuous importance long after the enclosure was first laid out.