Under the Anglo-Irish Treaty of 1921, three strategically important ports remained in British hands: Cobh (then called Queenstown) and Berehaven, both in County Cork, and Lough Swilly in County Donegal. Spike Island formed part of the Queenstown Harbour Defences.
Throughout the 1920s and up to 1938, Spike Island was manned by the Royal Artillery—Coast Defences, and the Signal Section.
In the middle of January 1938, the governments of Éire and the UK agreed to initiate formal talks in London. Éamon de Valera (‘An Taoiseach’—the Irish ‘Prime Minister’) was accompanied by Seán Lemass, James Ryan and Seán MacEntee. The talks centred on four areas of contention: partition, finance, trade and defence. After the inaugural discussions in London, the agreement was reached on 25 April 1938 and signed in the Cabinet Room at No. 10 Downing Street. On the defence side, the UK government agreed to transfer the coast defence stations occupied by the British naval and other forces to Éire under the Anglo-Irish Treaty of 1921.
Winston Churchill, who held no government office at the time of the handover of the Treaty Ports, spoke it against as an MP:
(…) When I read this Agreement in the newspapers a week ago I was filled with surprise. On the face of it, we seem to give everything away and receive nothing in return … But then I supposed there was another side to the Agreement, and that we were to be granted some facilities and rights in Southern Ireland in time of war. That, I notice, was the view taken by a part of the Press, but soon Mr. de Valera in the Dáil made it clear that he was under no obligations of any kind and, as the Prime Minister confirmed … On the contrary, Mr. de Valera has not even abandoned his claim for the incorporation of Ulster (…).
Eventually, under the Anglo-Irish Agreement of 1938, the transfer of the ‘Treaty Ports’ was finally approved.
The handover of Spike Island to the Irish Government took place on 11 July 1938, on the 17th anniversary of the 1921 Truce. The ceremony was attended by then Taoiseach Éamon de Valera and was watched across the harbour in Cobh by an estimated 40,000 people who cheered the raising of the Tricolour over Cork harbour forts for the first ever time.
Following the handover, Spike Island became an Irish Army based garrisoned by the Artillery Corps, Coastal Defence Artillery (CDA). During the Second World War (known In Ireland as ‘The Emergency’), Fort Westmoreland played a key role in the protection of the harbour.
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.