Treaty Ports

Following the establishment of the Irish Free State, three deep water Treaty Ports (Irish: Calafoirt an Chonartha) 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. 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.

Port data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).