Following the establishment of the Irish Free State, three deep water Treaty Ports (Irish: Calafoirt an Chonartha[1]) at Berehaven, Spike Island (off modern Cóbh), and Lough Swilly were retained by the United Kingdom in accordance with the Anglo-Irish Treaty of 6 December 1921.[2]
The main reason for the retention of the ports was the U-boat Campaign around Irish coasts during World War I and the concern of the British government that it might recur. As a part of the overall Anglo-Irish settlement, all other Royal Navy, British Army and RAF personnel and equipment were to evacuate the Free State.
As part of the settlement of the Anglo-Irish Trade War in the 1930s, the ports were transferred to Ireland (the Free State's successor) in 1938 following agreements reached between the British and Irish governments.