Known in Irish as a cillín (plural cilliní), these were informal burial places used for unbaptised infants, and sometimes for others considered ineligible for consecrated ground, including stillborn children, people who died by suicide, and occasionally strangers or travellers. The Catholic Church's position on original sin meant that unbaptised children could not, under older theological teaching, be buried in parish graveyards. And so families brought them instead to marginal places, the edges of fields, ancient earthworks, shorelines, and forgotten corners of the landscape.

Cilliní are found across Ireland in their hundreds, perhaps thousands, and Mayo has a particularly significant concentration of them. They tend to occupy places already felt to carry some spiritual weight, old ringforts, early medieval enclosures, or simply ground that stood apart from the everyday. The burials were carried out quietly, often at night, by the father alone or with a small group, without ceremony and without markers. That absence of formal commemoration is part of what makes these sites so difficult to study; they leave almost no material trace, and the knowledge of where they were was passed down informally, if at all. Many have been lost entirely, their locations forgotten within a generation or two after the practice ended, which it largely did during the twentieth century as theological attitudes shifted.