1937 Constitution:

On 29 December 1937, a new constitution, the Constitution of Ireland (Bunreacht na hÉireann), came into force. It replaced the Constitution of the Irish Free State and called the state Ireland, or Éire in Irish. The former Irish Free State government had taken steps to formally abolish the Office of Governor-General some months before the new Consti-tution came into force. Although the Constitution of Ireland established the office of President of Ireland, between 1937 and 1949 Ireland was not technically a republic. This was because the principal key role possessed by a head of state, that of symbolically representing Ireland internationally remained vested under statutory law, in the British king as an organ of the Irish government. The King's title in the Irish Free State was exactly the same as it was elsewhere in the British Empire:

1922-1927 - By the Grace of God, of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India.
1927-1937 - By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India.

Ireland remained neutral during World War II, a period it described as The Emergency. The position of King ceased with the passage of the Republic of Ireland Act 1948, which came into force on 18 April 1949 when the office of President of Ireland replaced that of the King. The Act declared that the state could be described as a republic. Later, the Crown of Ireland Act was formally repealed in Ireland by the Statute Law Revision (Pre-Union Irish Statutes) Act, 1962.

Ireland was technically a member of the British Commonwealth after independence until the declaration of a republic on 18 April 1949. Under the Commonwealth rules at the time, a declaration of a republic automatically terminated membership of the Commonwealth. This rule was changed 10 days after Ireland declared itself a republic, with the London Declaration of 28 April 1949. Ireland therefore immediately ceased to be a member and did not reapply for membership when the Commonwealth changed its rules to allow republics to join.