Under the Ango-Irish Treaty of 1921, forts in a number of Irish ports were designated to remain in British hands: these included Berehaven, Co. Cork, Cobh (Queenstown), Co. Cork, and Lough Swilly, Co. Donegal. Spike Island formed part of the Queenstown port defences. Spike Island was manned by the British Army throughout the 1920s and up to 1938. The British Government vehemently resisted handing over the ports to the Irish State but eventually, under the Anglo-Irish Agreement of 1938, their transfer was finally approved.
The transfer of Spike to the Irish Government took place on 11th July 1938, the anniversary of the Truce in 1921. The brief handover ceremony took place in the presence of An Taoiseach, Éamonn de Valera, and other senior members of the government including the Minister for Defence, Oscar Traynor, and the Minister for Finance, Dr. Jim Ryan. As soon as the last British soldiers had left the island, the tricolour was raised.
With the outbreak of World War II, the British hoped that the former ‘Treaty Ports’ would be made available to them as bases for the navy but despite strong diplomatic pressure, this never happened.
From 1938 to 1979, the Irish Army, especially the Artillery Corps, Coast Defence Artillery (C.D.A.), garrisoned Spike Island, primarily to man the sixinch guns. The two six-inch guns were re-located in 1943 to present locations. In 1979 the F.C.A. element of the C.D.A., became the 4th Air Defence Battery. The regulars became part of the First Field Artillery Regiment in Ballincollig.