Known in Irish as cillíní (singular cillín), these were the burial places reserved for unbaptised infants, and sometimes for others considered ineligible for consecrated ground, including suicide victims, strangers, and the unbaptised poor. The one at Ballyheer in County Mayo belongs to this tradition, a quiet patch of ground that carried enormous weight in the lives of the communities that used it.

The theology behind cillíní was stark. Catholic doctrine, as it was understood and practised in Ireland for centuries, held that unbaptised children could not enter heaven and therefore could not be buried in the churchyard alongside the baptised dead. Families who had lost an infant before baptism, or who had suffered a stillbirth, were left to find somewhere else. The places they chose were liminal by instinct: the margins of townlands, the edges of ancient ringforts, old ruined church sites, or simply a corner of a field with some prior sense of sanctity. These grounds were rarely formally marked, and burials were carried out quietly, often at night, by the father alone. The grief was real and deep, but it was largely private, because the loss sat outside the official rites of the community. In Mayo, where the land holds layer upon layer of pre-Christian and early Christian memory, such sites are not uncommon, and Ballyheer adds its own unnamed weight to that landscape.