The combination is quietly unsettling: the earthwork, which measures roughly 70 metres east to west and 60 metres north to south, was almost certainly old before the burials began, yet the two things ended up occupying the same ground, the prehistoric or early medieval structure becoming, in time, a place for the unbaptised dead.

The enclosure is defined by a bank of earth and stone, around two metres wide and one metre high, with an internal fosse, a ditch running on the inside of the bank rather than outside, which is about seven and a half metres across. This fosse runs from the north, around through the east, and down to the south, where it fades out; on the northwest to north side, the slope of the hillock itself appears to have been deliberately cut back, or scarped, to serve a similar defensive or boundary function. The overall form is that of a barrow, a broad category of earthen monument associated variously with burial, territorial marking, or enclosure, though the specific original use here is not recorded. The site is now very poorly preserved. A field boundary crosses the hillock near its western base, quarrying has eaten into the northern sector, and the southwest slope shows evidence of field-clearance material, the slow accumulation of stones moved off working land over generations. Each of these interventions has taken something from the monument's legibility.

The children's burial ground on the summit belongs to a tradition once widespread across Ireland, in which infants who died before baptism were interred in liminal places, outside consecrated ground but not quite in the ordinary landscape either. Ancient earthworks were frequently chosen for this purpose, perhaps because they already carried a sense of separateness, or because local memory attached significance to them without being able to fully explain it. At Ballygarraun, the hillock enclosed by the old bank seems to have served that function, with the summit becoming the place set apart for those the Church would not receive.