26 November 1948,
The Republic of Ireland proclaimed independence. The Irish Parliament passed the Republic Act, recognizing all the independence of the country. Tens of thousands of people took to the streets in Dublin to celebrate it and in public they lowered the UK flag and raised the Irish flag on the roof of Parliament. With this, southern Ireland broke its relations with Britain and the Commonwealth. The new republic entered the UN, while Northern Ireland remained dependent on the British.
Source:
Ulick O’Connor, Michael Collins and the Troubles. The Struggle for Irish Freedom 1912-1922 (W.W. Norton & Company).