Castledermot is situated in the south of the County Kildare on the banks of the river Lerr, whose name means river fork. According to the Annals of Innisfallen, it was founded in the year 812 by St Dermot, who was probably the grandson of Aedh Roinn, King of Ulster. Dermot was a member of the Culdee Reform Movement and established a monastery on the site of the present St James churchyard. Dermot was also described as a bishop and there were bishops in Castledermot until the church reform of the 11th Century. The monastery which was built here, was in part or total a Diseart and hence the place was called Diseart Diarmada, which means "Hermitage of Dermot.
As a testimony to its importance, the annals of early Irish history record many events that occured at Castledermot.