It derives from the Irish "cill", meaning a church or monastic cell, and wherever it appears it tends to signal an early Christian site, often long vanished above ground but preserved in the landscape through a graveyard that outlasted every other structure. The graveyard at Kill in County Mayo is one such place, where the burial ground itself becomes the only legible trace of a religious community that may stretch back over a thousand years.

Early Irish monasticism produced hundreds of small foundations, many of them modest affairs, a single oratory, a handful of cells, perhaps a simple enclosure. When these communities faded, through the upheavals of the medieval period or later plantation and consolidation, the graveyards frequently survived. Communities continued to bury their dead in ground already sanctified by generations of use, even after the buildings crumbled and the dedications were forgotten. A site named Kill in Mayo fits this pattern well, carrying the memory of a "cill" in its very name while the physical evidence of that original foundation has long since disappeared.