Following the secession of most of Ireland from the United Kingdom in 1922, the then created Irish Free State remained (for the purposes of British law) a dominion of the British Empire and thus its people remained as British subjects with the right to live and work in the United Kingdom and elsewhere in the Empire. The British monarch, as King in Ireland, continued to be head of state until the Statute of Westminster, 1931 granted the Irish Free State equal status with the United Kingdom and thereafter Ireland had its own monarchy in personal union with Britain and the other Dominions of the British Commonwealth (albeit with no distinct title, having the same title throughout the British Empire). However, by 1936, systematic attempts to remove references to the monarch from Irish constitutional law meant that the only functions remaining to the Crown were:
- signing Letters of Credence accrediting Irish ambassadors to other states; and
- signing international treaties on Ireland's behalf.
The Act's long title summarises the Act's several purposes:
An Act to recognise and declare the constitutional position as to the part of Ireland heretofore known as Eire, and to make provision as to the name by which it may be known and the manner in which the law is to apply in relation to it; to declare and affirm the constitutional position and the territorial integrity of Northern Ireland and to amend, as respects the Parliament of the United Kingdom, the law relating to the qualifications of electors in constituencies in Northern Ireland; and for purposes connected with the matters aforesaid.The effects of the Acts various subsections are as follows:
- s. 1(1) - Recognised that the country known in British law as "Eire" ceased to be "part of His Majesty's dominions" (i.e. a member of the Commonwealth) on 18 April 1949 (the date that the Irish "Republic of Ireland Act 1948" came into force). This provision also clarified that Ireland had remained a member of the Commonwealth until that date, as it had been held by some that Ireland was already a republic outside the Commonwealth.
- s. 1(2) – Declared that all of Northern Ireland would continue as part of the United Kingdom, and would remain within the Commonwealth, unless the Parliament of Northern Ireland consented otherwise.
- s. 1(3) – Established that the country up to then known in British law as "Eire" will in future be referred to by subsequent British legislation by the name "Republic of Ireland".
- s. 2(1) – Declared that, even though the Republic of Ireland was no longer a British dominion, it would not be treated as a foreign country for the purposes of British law.
- s. 2(2) – Established that the Irish ambassador to the United Kingdom would enjoy the same legal privileges with regard to taxation as the High Commissioners representing Commonwealth countries.
- s. 3(1) – Continued in force certain existing British laws in relation to the Republic of Ireland that had previously related to the Irish state.
- s. 3(2) – Made blanket provision for how certain wording in existing British legislation should be construed; for example, references to "His Majesty's dominions" were to be construed as including a reference to the Republic of Ireland despite its actual change of status.
- s. 3(3) – Excluded from the generality of the preceding subsection any requirement for the Oireachtas to assent to any change in the law relating to the succession to the throne or the royal style and titles; and also ended the requirement for the Irish government to receive notifications under the provisions of the Regency Act 1937.
- ss. 4 and 5 – Made certain technical provisions in relation to both transitional matters and to the citizenship of certain individuals born before the Irish Free State ceased to form part of the United Kingdom of Great Britain and Ireland.
- s. 6 – Made a number of technical changes to the electoral law relating to the election of MPs to sit at Westminster for Northern Ireland. The principal change meant required a voter to have lived in Northern Ireland for at least three months prior to the registration date; this change was introduced because the Government of Northern Ireland did not want people from the Republic coming to Northern Ireland to vote in elections there.
The Act created outrage in Ireland because its provisions guaranteed that partition (i.e. the status of Northern Ireland as a part of the UK) would continue unless the Northern Ireland parliament chose otherwise. Because Northern Ireland had a unionist majority, the guarantee that Northern Ireland would remain part of the UK unless the Belfast parliament resolved otherwise copper-fastened the so-called "unionist veto" in British law. The Irish parliament called for a Protest Against Partition as a result. This was the first and last cross-party declaration against partition by the Irish parliament. The revival of an Irish Republican Army in the early 1950s has been attributed by Irish journalist and popular historian Tim Pat Coogan to the strength of popular feeling among nationalists on both sides of the border against the Act.
Before the final Act was published, speculation that the legislation would change the name of "Northern Ireland" to "Ulster" was also the subject of adverse reaction from Nationalist politicians in Northern Ireland and from the Minister for Foreign Affairs in Ireland.
In general, a person born in what became the Republic of Ireland while it was still part of the United Kingdom before 6 December 1922 was not granted Citizenship of the UK & Colonies by the British Nationality Act 1948 unless such a person had a UK & Colonies born father (based on 1949 frontiers).
In order to acquire Citizenship of the UK and Colonies, such persons were expected to reside in the United Kingdom or a Crown Colony and acquire UK citizenship by registration or declaration.
Section 5 of the Ireland Act provided for acquisition of UK citizenship by some British subjects who had left what became the Republic of Ireland before it ceased to be part of the United Kingdom. Such persons generally became British citizens on 1 January 1983. See History of British nationality law
Persons not qualifying for this concession were nevertheless able to reclaim their British subject status under section 2 of the 1948 Act. This was later re-enacted as section 31 of the British Nationality Act 1981 and remains in effect as of 2012.