The Irish Oath of Allegiance was a controversial provision in the Anglo-Irish Treaty of 1921, which Irish TDs (members of the Irish parliament) and Senators were required to take, in order to take their seats in Dáil Éireann (The Chamber of Deputies) and Seanad Éireann (the Irish Senate).
The Oath was included in Article 17 of the Irish Free State's 1922 Constitution.
I ... do solemnly swear true faith and allegiance to the Constitution of the Irish Free State as by law established, and that I will be faithful to H.M. King George V, his heirs and succes-sors by law in virtue of the common citizenship of Ireland with Great Britain and her adherence to and membership of the group of nations forming the British Commonwealth of nations.
Such an Oath was typical within the Commonwealth, but in the Ireland of 1921-22 its intended effect was also to indicate some continuity to outside investors and help sustain the Irish economy.
The Oath had to be taken in front of the Governor-General of the Irish Free State or some other person authorised by him.
The Oath was widely condemned by the anti-treaty campaigners as involving Irish politicians taking an Oath of Allegiance to the British King. However, as the wording shows, that was an incorrect interpretation.
The Oath of Allegiance was actually 'to the Irish Free State as by law established (a line drafted by de Valera in his own proposed oath). The reference to the King involved a promise of fidelity, not an Oath of Allegiance.
The fidelity to the King was not to him as British monarch but in virtue of the common citizenship of Ireland with Great Britain and her adherence to and membership of the group of nations forming the British Commonwealth of nations, in other words, in his role as the symbol of the Treaty settlement, not as British King.
As the Oath was effectively to the elected government in the Irish Free State, it was also described as the Crown in Ireland. Opposition to this was based on the fact that it was not fully discussed and explained before the Anglo-Irish Treaty was signed in December 1921, and that many of the members of the second Dáil Éireann, elected without opposition in May 1921, had already sworn an Oath to uphold an Irish Republic.
While the Republican Oath was much mentioned in the Treaty debates of 1921-22, it had taken over a year to arrange to have the oath sworn by the Dáil TDs and IRA volunteers, between May 1919 and August 1920. It then became a suitably symbolic reason to oppose the Treaty.
Ironically, in view of the opposition expressed to the Oath by anti-treatyites, it was in fact largely the work of Michael Collins, based in its open lines on a draft oath suggested by the President of the Republic, Éamon de Valera, and also on the oath of the Irish Republican Brotherhood. In fact, Collins cleared the Oath with the IRB before proposing it during the Treaty negotiations. By the standards of the Oaths of Allegiance to be found in other British Commonwealth dominions, it was quite mild, with no direct personal Oath to the monarch, only an indirect oath of fidelity by virtue of the King's role in the Treaty settlement as "King in Ireland", a figurehead position. However mild it was, the public perception among those who were hostile to the Treaty was that it was an offensive Oath to the British monarch. The problem with the Oath wasn't its actual words, but the perspective formed on it.
De Valera and abolition:
When de Valera founded Fianna Fáil as the party of an "Irish Republic" in 1926, he and his party, though agreeing to contest elections, refused to take the Oath. However the assas-sination of the Vice-President of the Executive Council (deputy prime minister), Kevin O'Higgins, led the Cumann na nGaedhael government under W. T. Cosgrave to introduce a law requiring all Dáil candidates to promise that they would take the Oath. Otherwise they could not contest the election. Backed into a corner, de Valera took the Oath, declaring that he was simply signing a piece of paper to be admitted to the Dáil. In power from 1932, de Valera amended the Free State's constitution firstly to allow him to introduce any constitutional amendments irrespective of whether they clashed with the Anglo-Irish Treaty, then amended the constitution to remove Article 17 of the constitution which required the taking of the Oath. Ironically it was the political descendants of Michael Collins, the Fine Gael-pro-treaty party in a coalition with an anti-treaty party, not de Valera of the Fianna Fáil-who did declare the state to be a republic in 1948.