Old Leighlin (Irish: Seanleithghlinn) is a small town in County Carlow, Ireland, 3.5 km west of Leighlinbridge. The site was at one time one of the foremost monastic houses in Leinster, with 1500 monks in residence. It was the location for a church synod in AD 630, which decided the date on which Easter Day would fall for the entire Christian world.