https://en.wikipedia.org/wiki/Republic_of_Ireland
After the War of Independence and truce called in July 1921, representatives of the British government and the five Irish treaty delegates, led by Arthur Griffith, Robert Barton and Michael Collins, negotiated the Anglo-Irish Treaty in London from 11 October to 6 December 1921. The Irish delegates set up headquarters at Hans Place in Knightsbridge, and it was here in private discussions that the decision was taken on 5 December to recommend the treaty to Dáil Éireann. On 7 January 1922, the Second Dáil ratified the Treaty by 64 votes to 57.[40]
In accordance with the treaty, on 6 December 1922 the entire island of Ireland became a self-governing Dominion called the Irish Free State (Saorstát Éireann). Under the Constitution of the Irish Free State, the Parliament of Northern Ireland had the option to leave the Irish Free State one month later and return to the United Kingdom. During the intervening period, the powers of the Parliament of the Irish Free State and Executive Council of the Irish Free State did not extend to Northern Ireland. Northern Ireland exercised its right under the treaty to leave the new Dominion and rejoined the United Kingdom on 8 December 1922. It did so by making an address to the King requesting, "that the powers of the Parliament and Government of the Irish Free State shall no longer extend to Northern Ireland."
