(In this article 'Ireland', unless otherwise stated, refers to the Republic of Ireland.)
From 1 January 1801 until 6 December 1922 Ireland was part of the United Kingdom of Great Britain and Ireland. Historically it had for centuries been governed as one all-island unit. This changed with the the introduction of partition in the British Government of Ireland Act, 1920. This created two states, Southern Ireland (of 26 counties) and Northern Ireland (of 6 counties), both of which were to remain part of the United Kingdom. While Northern Ireland became a political reality, Southern Ireland initially existed only on paper, its governing institutions never having come into being.
From 1919 to 1922 a UDI all-island state called the Irish Republic nominally existed, having been declared by the First Dáil, an illegal 'Assembly of Ireland' set up by Irish politicians who had been elected to sit in the British House of Commons but who had declined to do so, setting up a rival parliament instead. Though unrecognised internationally, the Irish Republic functioned in a haphazard manner as a rival government with its own prime minister (later upgraded to become President of the Republic) and a cabinet. Its army, the Irish Republican Army, waged a guerrilla war against the British Army and police force, in what came to be known as the Anglo-Irish War (also known as the Irish War of Independence).
In December 1921, the British Government and Irish Republican plenipotentiaries[?] negotiated a peace treaty, known as the Anglo-Irish Treaty. It created a whole new system of Irish self government, known as dominion status[?], with a new state, to be called the Irish Free State (in the Irish language Saorstát Éireann). The new Free State was in theory to cover the entire island, subject to the provisio that Northern Ireland could opt out and choose to remain part of the United Kingdom, which it duly did. For one year, Southern Ireland, which had previously existed only on paper, was resurrected and governed by a cabinet under Michael Collins. (After his assassination in August 1922 W.T. Cosgrave assumed control.) The Irish Republic in theory continued to exist, with both Southern Ireland and Irish Republic disappearing similtaneously and being replaced by the new Irish Free State on 6 December 1922. In the absence of the six counties of Northern Ireland, the new state, which was independent of the United Kingdom, covered twenty-six of the island's thirty-two counties.
The Irish Free State was a constitutional monarchy over which the British monarch reigned (from 1927 with the title King of Ireland). The Representative of the Crown was known as the Governor-General of the Irish Free State. It had a bicameral[?] parliament and a cabinet, called the Executive Council answerable to the Chamber of Deputies, which was known as Dáil Éireann. The prime minister of the Free State was called the President of the Executive Council. The constitution was called the Irish Free State Constitution.
On the 29 December 1937 a new constitution came into being. It replaced the Irish Free State by a new state called Éire. The Governor-General was replaced by a President of Ireland. A new more powerful prime minister, called the Taoiseach came into being, while the Executive Council was renamed the Government. Though it had a president, the new state was not a republic. The British monarch continued to reign as King of Ireland and was used as an "organ" in international and diplomatic relations, with the President of Ireland relegated to symbolic functions within the state but never outside it.
On 1 April 1949, the Republic of Ireland Act, came into force. The new state was unambiguously described as a republic, with the international and diplomatic functions previously vested in or exercised by the King now vested in the President of Ireland who finally became unambiguously the Irish head of state. Though the official name of the state remained Éire, the term Republic of Ireland though officially just the description of the new state, came to be used as its name. While the Republic often chose to use the word Ireland to describe itself, particularly in the diplomatic sphere, many states avoid using that term because of the existence of a second Ireland, Northern Ireland, and because the 1937 constitution claimed that the south had jurisdiction over the north. Using the word 'Ireland' was taken as accepting that claim and so caused offence in Northern Ireland. That claim, in what was known as Articles 2 and 3 of the 1937 constitution, was repealed in 1999.
The Irish Free State/Éire remained a member of the British Commonwealth until the declaration of a republic in April 1949. Under Commonwealth rules, declaration of a republic automatically terminates membership of the Commonweath. Unlike India, which became a republic at the same time, the Republic of Ireland chose not to reapply for admittance to the Commonweath.
For more on Irish history, see the History of Ireland page.