These sites, known in Irish as cillíní, were set apart from consecrated parish graveyards because Catholic doctrine long held that unbaptised children could not be buried in hallowed ground. Families instead brought their infants to older, liminal places, often early medieval enclosures or ruined churches whose sanctity was felt without being officially recognised by the Church. Ballynabrennagh is one such place, layered with uses that span well over a thousand years.

The 1841 edition of the Ordnance Survey six-inch map recorded a small rectangular church standing in the southern portion of a circular earthwork, the latter shown with a dotted outline suggesting it was already partially obscured by that point. The Ordnance Survey Name Books, compiled around the same period, describe the church walls as standing roughly three feet high, placed within a circular area approximately two chains in diameter, that is, around forty metres across, and enclosed by a fosse or old stone ditch. A fosse is simply a ditch, often part of an original boundary or defensive feature. The Name Books also noted that infants were said to be sometimes interred there, capturing the site mid-practice rather than in retrospect. Nearly a century later, the Minute Book of the County Kerry Field Club, dated 2 March 1940, recorded that human bones had been found in the field containing both the church and the enclosure. The earthwork itself may originally have functioned as the boundary ditch of an early Christian burial ground associated with the church, a space that was later quietly repurposed for the burial of the unbaptised when formal churchyards were no longer available to them.