Old Leighlin was a constituency represented in the Irish House of Commons, the lower house in the Irish Parliament of the Kingdom of Ireland. It based in Old Leighlin, near the town of Leighlinbridge in County Carlow. It was a bishop's borough, controlled by the Church of Ireland Bishop of Ferns and Leighlin. The borough was disenfranchised by the Acts of Union 1800, with effect from 1 January 1801.