Old Leighlin, also Oldleighlin, is a village, civil parish and townland in County Carlow, Ireland. The village is 3.5 km (2 mi) west of Leighlinbridge. The site was at one time one of the foremost monastic houses in Leinster, with 1,500 monks in residence. It was the location for a church synod in AD 630, which decided that the Irish church should follow Roman as opposed to Celtic dating conventions for determining the date of Easter.