Curving around its southern and western edges is a low bank of earth and stone, roughly two and a half metres wide and a metre high, that traces the boundary of an early ecclesiastical enclosure, the kind of roughly circular or oval perimeter that early Christian communities in Ireland used to mark off sacred ground from the surrounding landscape. What makes Killegar quietly unusual is that two distinct phases of enclosure seem to be visible here: a smaller, roughly circular feature of about fifty metres in diameter, and a larger one measuring approximately eighty metres north to south and a hundred metres east to west.

The smaller enclosure was already recorded on the Ordnance Survey's six-inch map of 1838, suggesting it was a legible feature in the landscape even then, though only a portion of it now survives to the west of the present graveyard. The larger enclosure is defined by that surviving curving bank, running from the southern boundary of the modern graveyard around to the west. To the north and east, however, the feature has been lost entirely to quarrying, which makes it difficult to read the full original shape. Early ecclesiastical enclosures of this type are generally associated with monastic or proto-monastic activity in early medieval Ireland, and their boundaries often endured for centuries in the form of field walls, roads, or, as here, graveyard margins that quietly preserved the line of something far older.