The town of Kildare takes its name from an oak tree. Cill Dara, meaning 'the church of the oak', referred to a tree that the seventh-century writer Cogitosus placed beside St. Brigid's monastery when it was founded in the early sixth century. That a modern county town should carry the memory of a single tree in its name is quietly remarkable, and it points to how thoroughly the monastery shaped everything that came after it.

Brigid's foundation began as a house for nuns, but it evolved into what is known as a double monastery, a form of early medieval religious community in which nuns under an abbess and monks under an abbot or bishop occupied the same complex, typically in separate but adjoining spaces. Cogitosus, writing his Life of Brigid around 680, describes a large church with many windows and decorated doorways, the nuns and monks separated inside by a screen running the length of the building. The altar was screened off as well, the partition hung with paintings and cloth, and on either side of it stood the richly ornamented shrines of Brigid and Conleth, the first recorded bishop, who died in 520 according to the Annals of Ulster. Those shrines were reputedly carried off by Viking raiders in 835. The monastery burned repeatedly between 710 and 1089, not always through hostile action. A thunderbolt in 1012, recorded in the Annals of Clonmacnoise, destroyed 'all the Towen of Kildare but one house', which tells us that a substantial secular settlement had grown up around the religious core by that point. A fire in 1050 took both the wooden church, known in Irish as a durthech, and the stone church, or damhlaig. By 1111, at the Synod of Rathbreasail, Kildare was formally recognised as one of the five episcopal sees of Leinster.

The precise footprint of the early monastery cannot be fixed with certainty, but the evidence points strongly to the area around the existing cathedral, high cross, and round tower in the graveyard. More subtly, some of the curving streets of the present town may follow the outlines of early monastic enclosures, the original boundaries having bent the grain of the settlement so gradually that their shape persisted long after the walls themselves were gone.