6 December 1922: The Foundation of the Irish Free State OTD. It came into existence as a result of the Anglo- Irish Treaty aka ‘the Treaty’ that was signed in London exactly one year previously. It placed the new polity on a full legal footing with the other Dominions within the British Empire, namely Australia, Canada, New Zealand & South Africa. 

It gave the Irish Free State/ (Saorstát Éireann) full international status on the world stage and the ability to [more or less] conduct its own affairs within its territorial boundaries that comprised 26 of Ireland’s 32 counties. However it’s signing caused deep rifts within Nationalist Ireland that led to Civil War that was still raging when it came into final effect.

There is no doubt that the majority of the people in the South did favour accepting the Treaty. It gave them most of what they wanted, basically peace and a large measure of independence. The Catholic Church, the large farmers, the newspapers and the business community were its strongest supporters. The Labour movement was also largely in favour of acceptance. It was incomprehensible to the many supporters of the Treaty as to why it should be rejected. They argued that to all intents and purposes it allowed most of Ireland to manage her own affairs. 

To quite a significant minority though, especially amongst the ranks of the Irish Republican Army, the Treaty was unacceptable. The Irish had signed the agreement under the threat of the immediate renewal of war by the British, if the Irish delegates turned down its terms. It recognised Partition and worst of all an Oath of fidelity to King George V of England would mean a rejection of the Irish Republic, which they had fought for and which many of their comrades had died for.

The flaws within the document were chipped away over the years that followed...

In 1936, against the backdrop of the Abdication Crisis in Britain, de Valera authored a new Constitution stripped of any reference to the British Crown and on 29 December 1937, when Bunreacht na hÉireann came into operation, the Irish Free State was succeeded by Éire (Ireland), a 'sovereign, independent, democratic state' – a republic in all but name. 

https://www.rte.ie/history/irish-free-state/2022/1124/1338099-the-irish-free-state/

An actual Republic was publicly announced in 1948 by An Taoiseach John A. Costello and was signed into law on 21 December 1948. It came into force on 18 April 1949, Easter Monday - the 33rd anniversary of the start of the Easter Rising. That is still the legal position to this day notwithstanding the State being within the EU and the Anglo- Irish Agreement of 1998.