These were informal, unconsecrated plots used for centuries to inter unbaptised infants, and sometimes others excluded from formal Catholic burial, including stillborn children, women who died in childbirth, and occasionally strangers or suicides. Because the Catholic Church long held that baptism was a prerequisite for burial in consecrated ground, families were left to find their own quiet corners of the landscape, often at the margins of fields, beside ancient earthworks, at the edges of bogs, or close to pre-Christian monuments. The choice of location was rarely random; many cillíní occupy places already understood as liminal or set apart.

Thousands of these sites are scattered across Ireland, and Mayo has a particularly high concentration of them, a reflection of both the county's historically dense rural population and its strong oral traditions around the care of the unbaptised dead. The practice of burying children in cillíní persisted well into the twentieth century in some parts of the west of Ireland, long after it had faded elsewhere. The grief attached to these places was often private and unrecorded, which is part of why so many sites remain poorly documented. An Ard Eoghain, like many such townlands in Connacht, carries a name rooted in early Gaelic geography, and the presence of a cillín there fits a pattern common across the region, where layers of pre-Christian, early Christian, and post-medieval land use overlap in ways that formal historical records rarely capture fully.