The Ireland Act 1949 (12, 13 & 14 Geo. 6. c. 41) is an act of the Parliament of the United Kingdom intended to deal with the consequences of the Republic of Ireland Act 1948 as passed by the Irish parliament, the Oireachtas.

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 British subjects with the right to live and work in the United Kingdom and elsewhere in the Empire. The British monarch continued to be head of state. 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:

This status quo remained, with Ireland participating little in the British Commonwealth and Éamon de Valera remarking in 1945 that "we are a republic" in reply to the question if he planned to declare Ireland as a republic. Then somewhat unexpectedly in 1948, during a visit to Canada, Taoiseach John A. Costello announced that Ireland was to be declared a republic. The subsequent Irish legislation, the Republic of Ireland Act 1948, provided for the abolition of the last remaining functions of the King in relation to Ireland and provided that the President of Ireland would exercise these functions in the King's place. When the act came into force on 18 April 1949, it effectively ended Ireland's status as a British dominion. As a consequence of this, it also had the effect of ending Ireland's membership in the British Commonwealth of Nations and the existing basis upon which Ireland and its citizens were treated in the United Kingdom and other Commonwealth countries as "British subjects", not foreigners.

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:

The Ireland Act was also used by the United Kingdom to "repair an omission in the British Nationality Act, 1948". The British Nationality Act included provisions dealing specifically with the position of "a person who was a British subject and a citizen of Eire on 31st December, 1948". Because of this, how the British law would apply was dependent on a question of Irish law, namely, who was a "citizen of Eire"? The UK Government seriously misunderstood the position under Irish law. The UK Secretary of State for Home Affairs explained that:

[w]hen British Nationality Act was passed the United Kingdom did not know that a person who was born in Southern Ireland of a Southern Irish father and on 6th December, 1922, was domiciled in Northern Ireland was regarded as a citizen of Eire under Eire law. They therefore considered that such a person became a United Kingdom citizen on 1st January, 1949 [in accordance with the British Nationality Act].