These were informal, unconsecrated burial places used for centuries to inter unbaptised infants, stillborn children, and occasionally others considered ineligible for burial in sanctified ground. The practice arose from Catholic teaching that held unbaptised souls in a state of limbo, excluded from the rites and spaces reserved for the baptised dead. Families, reluctant to leave their children in entirely unmarked ground, turned instead to liminal places, old ringforts, field boundaries, shorelines, and pre-Christian enclosures, places already set apart from ordinary use.

Kilroughil itself carries an element of ecclesiastical history in its name, suggesting proximity to an early Christian site, and children's burial grounds of this type are frequently found near the ruins of early churches or within the earthworks of much older enclosures. The association was partly practical and partly freighted with older ideas about sacred ground that predated the formal church calendar. These sites were used well into the twentieth century in some parts of Ireland, long after official disapproval had made them socially fraught. The grief surrounding them was largely private, the burials conducted quietly and without ceremony, which is partly why so many cilliní remain poorly documented and only partially mapped.