It measures less than a metre across and barely reaches ankle height, yet the circular hollow ground into its upper surface places it within a category of object that has puzzled and fascinated archaeologists for generations. This is a bullaun stone, a term for a boulder or slab bearing one or more cup-shaped depressions worn or worked into the surface. Their precise purpose remains contested; theories range from the practical, such as grinding or mortaring, to the votive, with some bullauns associated with healing rituals or the collection of sacred water.

The stone at Kilnaruane sits within an early ecclesiastical enclosure, the kind of defined sacred boundary that characterised early Irish monastic and church sites, often marked by a curving bank or ditch enclosing the church, its associated buildings, and the burial ground. The bullaun itself is sub-rectangular in shape and modest in scale, roughly 0.75 metres by 0.60 metres and only about 0.15 metres high. The hollow on its upper surface is centrally placed, circular, approximately 0.3 metres in diameter and 0.1 metres deep. These precise, almost formal proportions suggest deliberate workmanship rather than natural weathering, though the stone gives nothing away about who made it or when.