A killeen, sometimes spelled cillin, is an unconsecrated burial ground, typically used for unbaptised infants, stillborns, and occasionally others considered ineligible for burial in sanctified earth, such as suicides or strangers. These sites exist in their hundreds across Ireland, often marked by little more than a scatter of small stones or a slight unevenness in a field, their locations preserved mainly in local memory and placename evidence rather than in any formal record.

The name itself carries the history. Killeen derives from the Irish cillín, a diminutive of cill, meaning a small church or burial ground, and the prefix in a placename is often the only surviving indicator that a site of this kind once served a community. In the pre-Famine and post-Famine countryside of Mayo, where infant mortality was high and the institutional reach of the Church was absolute, these informal grounds occupied a sorrowful middle ground between the sacred and the secular. Families who could not bury their children in consecrated ground brought them instead to these marginal spaces, often at night and without ceremony, to places already associated with the ancient or the liminal, including old ringfort boundaries, coastal edges, and forgotten enclosures.