These grounds, known in Irish as cillíní (singular: cillín), were informal burial places set aside for unbaptised infants, and sometimes for others considered ineligible for consecrated ground, including stillborn children, suicides, and strangers. Their locations were often marginal in a deliberate sense: field boundaries, coastal edges, the ruins of early medieval enclosures, or simply a quiet corner of land apart from the parish cemetery. The choice of place was rarely arbitrary, and many cillíní were associated with pre-Christian or early Christian sites, suggesting a layered relationship between older sacred landscapes and later Catholic practice.

The practice of burying unbaptised children separately was bound up with the doctrine of limbo, which held that those who died without baptism could not enter heaven. For families, it meant burying an infant without funeral rites, often at night and with little ceremony, in ground that carried its own quiet significance even if it lacked official sanction. The grief attached to these places was largely unspoken in public life for generations, and cillíní remained poorly documented as a result. In recent decades, Irish archaeology and folklore studies have worked to record and understand them more fully, recognising that they represent an important, if sombre, dimension of how ordinary communities managed death, loss, and religious obligation across several centuries.