The churchyard at Kill, Co. Kildare holds rather more history than its modest surroundings suggest. Tucked into the graveyard to the south of the Church of Ireland building is a chamfered window jambstone, complete with a glazing groove and bar holes, salvaged from a medieval church that has otherwise left no trace above ground. Beside the north door of St John's Church sits a font of possible 16th-century date, slightly out of place, a survivor without a building. The church itself is gone; what remains are these quiet fragments, and the knowledge that several layers of religious activity lie beneath the present ground surface.

The place name Kill derives from the Irish "cill", meaning church or monastic cell, and the settlement here is thought to have begun as an early monastic foundation, though no surface evidence of that monastery has survived. Scholars have suggested it lay on or close to the site of the 19th-century St John's Church. In 1993, pre-development archaeological test-trenching revealed part of the enclosing fosse, a boundary ditch that would have defined a probable ecclesiastical enclosure, confirming that something significant once occupied this ground. Within that enclosure stood a later Anglo-Norman church dedicated to Saints Mary and Brigid, which had been granted to St Thomas' Abbey in Dublin by 1202. References to Kill church in several 13th-century grants suggest it carried real institutional weight at that time, though its later medieval story is largely obscure. It remained connected to St Thomas' Abbey as late as 1540, and continued in use even after the Dissolution of the Monasteries, when so many similar foundations fell silent. The present St John's Church was most likely built directly on the footprint of that medieval structure.