Under the constitution of the Irish Free State The King of the United Kingdom was the their Head of State as that constitution established the Irish Free State as a constitutional monarchy. However, with the new constitution of 1936 until April 1949 it was unclear whether the Irish state was a republic or a form of constitutional monarchy and (from 1937) whether its head of state was the President of Ireland or King George VI.

The exact constitutional status of the state during this period has been a matter of scholarly and political dispute. The Oireachtas removed all references to the monarch from the revised constitution in 1936, but under statute law the British monarch continued to play a role in foreign relations, though always on the advice of the Irish government. The state did not officially describe itself as the Republic of Ireland until 1949, when it passed legislation giving itself that description.
Background
The state known today as Ireland is the successor-state to the Irish Free State which was established in December 1922. The Irish Free State was governed, until at least 1936, under a form of constitutional monarchy. Under the Free State’s constitution the King had a number of nominal duties, including exercising the executive authority of the state, appointing the cabinet and promulgating the law. However, all of these were delegated to the Governor-General of the Irish Free State, and in 1927 the King’s title within Great Britain and Northern Ireland was changed by proclamation under the Royal and Parliamentary Titles Act passed by the Westminster Parliament to “George V, by the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India”.
King’s title in the Irish state
The King’s title in the Irish Free State (1922–1937) and in Ireland (1937–1949) was the same as it was elsewhere in the Commonwealth, being:
* 1922–1927: By the Grace of God, of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India
* 1927–1948: By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India
* 1948–1949: By the Grace of God, of Great Britain, Ireland and the British Dominions beyond the Seas King, Defender of the Faith.
Edward VIII and the abdication crisis

Edward VIII, King of the United Kingdom of Great Britain and Ireland, Emperor of India
In January 1936, George V died and was succeeded by his eldest son, who became Edward VIII. The new King’s reign lasted only eleven months, and he abdicated in December of that year and was succeeded by his brother Prince Albert, Duke of York, who took the name George VI. The parliaments of independent members of the British Commonwealth were required to ratify this change in monarch, and the pro-republican government of the Irish Free State decided to use this opportunity to drastically change the constitution.
Immediate post-abdication reforms (1936–37)
The day after the abdication was announced on December 12, 1936, the Free State constitution was amended to remove all mention of the King and abolish the office of governor-general. The following day, a separate statute permitted the King to sign international treaties and to accredit diplomatic representatives, where authorised by the Irish government.
In 1937 a new Constitution was adopted establishing the monarch’s diminished role, transferring many of the functions performed by the King until 1936 to a new office of President of Ireland, who was declared to “take precedence over all other persons in the State”. However, the 1937 constitution did not explicitly declare that the state was a republic, nor that the President was head of state, and it allowed for the King to have a role in the state’s external affairs. The state’s ambiguous status ended in 1949, when the Republic of Ireland Act ended the King’s remaining role in external affairs and declared that the state was a republic.
The status of the Head of the Irish State from 1936 to 1949 was largely a matter of symbolism and had little practical significance. This was because the roles of both the King and the President of Ireland were merely ceremonial, being exercisable only “on the advice” of the government (Cabinet). However, one practical implication of explicitly declaring the state to be a republic in 1949 was that it automatically led to the state’s termination of membership of the then British Commonwealth, in accordance with the rules in operation at the time.
Constitution of 1937
The Constitution of Ireland, adopted in 1937, filled the gap left by the abolition of the governor-general by creating the post of a directly elected president. The President of Ireland was henceforth responsible for the ceremonial functions of dissolving the legislature, appointing the government, and promulgating the law. Unlike most heads of state in parliamentary systems, the President was not even the nominal chief executive. Instead, the role of exercising executive authority was explicitly granted to the government—in practice, to the Taoiseach. The constitution also, like the 1922 constitution that preceded it, contained many provisions typical of those found in republican constitutions, stating, for example, that sovereignty resided in the people and prohibiting the granting of titles of nobility.

George VI, King of the United Kingdom of Great Britain and Ireland, Emperor of India
Nonetheless the government of Éamon de Valera, despite its long-term goal of republicanising the Irish state, consciously chose not to declare a republic and decided to name the state simply Éire (or Ireland), rather than the “Republic of Ireland” or the “Irish Republic”. Thus the new constitution did not explicitly declare that the President would be head of state, providing merely that he would “take precedence over all other persons in the State”. Nor did the new document mention the word republic. Most crucially, Article 29 of the new constitution mirrored Article 51 of its predecessor, by permitting the state to allow its external relations to be exercised by the King.
Nonetheless from 1936 until 1949 the role of the King in the Irish state was invisible to most Irish people. The monarch never visited the state during that period and, due to the abolition of the office of governor-general, had no official representative there. The President, on the other hand, played a key role in important public ceremonies.
Asked to explain the country’s status in 1945, de Valera insisted that it was a republic.
Republic of Ireland Act
The Republic of Ireland Act 1948, which came into force on April 18, 1949, the 33rd anniversary of the beginning of the Easter Rising, was remarkable in that it purported to reform the state into a republic without making any change to the constitution, the ambiguous provisions of which remained unaltered. The Republic of Ireland Act contained three major provisions; it declared that: the External Relations Act was repealed, the state was a republic, and the external relations of the state would henceforth be exercised by the President. The act also had the effect of automatically terminating the state’s membership of the Commonwealth.
Soon after President Seán T. O’Kelly signed the act into law, he commemorated his new status as the clear and unambiguous Irish head of state with state visits to the Holy See and France. A visit to meet George VI at Buckingham Palace was also provisionally planned, but timetabling problems with the President’s schedule prevented the meeting.
Outside the Irish state, “Great Britain, Ireland” was not officially omitted in the royal title until 1953 in the reign of Elizabeth II. Then, each Commonwealth realm adopted a unique title for the monarch. No mention of Ireland was made in any except in the title within the United Kingdom and its dependent territories: it was changed from “of Great Britain, Ireland and the British Dominions beyond the Seas, Queen” to “of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories, Queen”.
In 1962 the Republic of Ireland officially repealed Crown of Ireland Act of 1542.

Elizabeth II, Queen of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories.