These were informal, unconsecrated plots where, for centuries, unbaptised infants were quietly interred outside the boundaries of sanctioned parish cemeteries. Catholic doctrine, as it was long practised, held that a child who died before baptism could not be buried in consecrated ground, and so communities found their own places: old ringforts, abandoned monastery sites, field margins, coastal dunes, or simply a corner of land set apart by quiet local agreement. The practice continued in some areas well into the twentieth century, and the sites themselves are often unassuming to the point of near-invisibility, marked only by small stones or slight undulations in a field.

Cillíní are found in considerable numbers across the west of Ireland, and Mayo has a particular concentration of them, a reflection of both the county's strong folk-Catholic traditions and its historical geography. The grief that surrounds these places was, for a long time, a largely private matter. Families who buried children in cillíní rarely had access to formal mourning rituals, and the sites were maintained through local memory rather than institutional record. That memory has faded in many places, and a number of these grounds have become difficult to locate or identify without local knowledge. The one at Cashel is recorded as a monument, which at minimum confirms that it was recognised as a site of cultural and archaeological significance, even if the details of its history remain thinly documented at present.