Kildare takes its name, Cill Dara, from a church built beside or upon a great oak, and tradition places its founding by Brigid in the last years of the fifth century or the first years of the sixth. Cogitosus, writing a Life of Brigit within about a century and a half of her traditional death, already describes a substantial double monastery there, housing both nuns and monks under Brigid's authority and a bishop she is said to have appointed, with a decorated church large enough to hold crowds of pilgrims and a screen dividing the sexes at worship.
The site he describes was already a major pilgrimage center by the time he wrote, drawing visitors from across Ireland and, he claims, from foreign kingdoms as well, largely on the strength of miracles associated with Brigid's relics kept there. Kildare's bishops and abbesses went on to exercise real authority over a wide federation of churches through the early medieval period, a position comparable to Armagh's claim in the Patrician tradition, and the monastery's scriptorium is credited with an illuminated gospel book, since lost, that Gerald of Wales in the twelfth century called more beautiful than any other book he had seen, comparing its intricacy to angelic rather than human workmanship.