These grounds, known in Irish as cilliní (singular cillín), were used for the interment of unbaptised infants, and sometimes others considered to exist outside the formal rites of the Catholic Church, including stillborn children, the unbaptised, and occasionally suicides or strangers. Because Church law historically prohibited such individuals from burial in consecrated ground, families turned to marginal spaces instead: old ringforts, the edges of fields, early medieval enclosures, or simply a corner of land felt to carry some older sanctity. The result is a scattered geography of small, often unmarked plots that represent one of the more quietly sorrowful aspects of post-medieval Irish rural life.

The practice of using cilliní spans several centuries, with many sites remaining in use well into the twentieth century. The locations chosen were rarely random. Families often selected places already associated with pre-Christian or early Christian activity, lending the site a sense of spiritual legitimacy that lay outside, but perhaps alongside, official Church authority. The Ardkill site fits within this broader tradition, sitting in a part of Mayo where the landscape retains numerous traces of earlier occupation and where the distances between communities and parish churches could make formal burial arrangements difficult, particularly for an infant who died before baptism could be administered.