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.
St Laserian's Cathedral was the cathedral of the diocese of Leighlin, now merged with neighbouring dioceses in the Church of Ireland. It is named after Molaise of Leighlin and was built on the site of an old monastic church founded here in 632 AD. It is one of the smallest Irish medieval cathedrals.
Old Leighlin gave its name to a constituency in the pre-1800 Parliament of Ireland, it was a Bishop’s boroughs whereby the local Church of Ireland Bishop selected the Member of Parliament.
Coordinates: 52°44′11″N 7°1′10″W / 52.73639°N 7.01944°W