A cillín (from the Irish language, with the literal meaning "little cell", "little churchyard" or "little burial ground"; plural cillíní), was a historical unconsecrated burial place in Ireland for children unbaptised at the time of death. Suicides, shipwrecked sailors, strangers, urepentant murderers and their victims were also sometimes buried there—they were used for "infants and other ambiguous categories of individual". Some of them are more than a thousand years old. Ancient pagan burial practices were sometimes later co-opted by Christianity.
The word cillín is a common element in Irish place names, often anglicised as Killeen. An alternative meaning of cillín indicates a small church, from the diminutive form of , meaning church. The word is thought to come from the , meaning little church or oratory. Another meaning for the word cillín is "cell" as in prison cell or monastic cell (many placenames are derived from the cell of a local monk/saint).