The President of Ireland is the Irish head of state.
The President has a largly ceremonial role with relatively little real political power.
Before the adoption of the 1937 constitution the Irish Free State had the British monarch also as its monarch and head of state. In exercising their role in Ireland, in particular from the end of the 1920s, Kings George V, Edward VIII and George VI were unambiguously doing so as King of Ireland, with no role whatsoever for the British state, British government, British Crown or even from 1931 the British Great Seal of the Realm which the Irish replaced with their own Seal (on which the Irish King appeared with the harp and the words 'Saorstát Éireann)'. The person who wore the Irish and British Crowns may have been the same, but in law they were different entities, as shown when from 1931 not merely did British ministers not have to be present at meetings between Irish ministers and the King, they were barred from attendance, to their fury.
It is a matter of considerable historical, legal and political debate as to who was Irish head of state between 1936/7 and 1949. For the functions normally performed by a head of state were spread over three different elements by the new constitution and statute law; the President of Ireland, the King of Ireland (an office created by the Royal Titles Act) and the Government of Ireland. The President was the state's 'first citizen.' Executive authority, which in most constitutional systems is vested in the head of state, in Bunreacht na hÉireann is vested in the Government, while the role of representing the Irish state abroad (signing treaties, accrediting ambassadors, receiving credentials from ambassadors to Ireland, etc) were exercised by the King of Ireland under Section 3 of the External Relations Act, 1936.
Generally, the latter function, of representing the state in international diplomacy, is presumed to be the key defining characteristic of a head of state. As a result, almost every state with which Éire (as Ireland is formally described in Article 4 of Bunreacht na hÉireann) had diplomatic relations with between 1937 and 1949 concluded that the Irish head of state was the man proclaimed King of Ireland in December 1936, King George VI. This view was echoed by then taoiseach John A. Costello in a debate in Seanad Éireann (the Irish senate) in December 1948, where he stressed the fact that the Republic of Ireland Act he was introducing would make Irish head of state the man who ought to have been but wasn't, the President of Ireland. Until the Republic of Ireland came into force in April 1949, the President of Ireland had no international role, and such an inferior status that he never dared set foot outside the state. The fact that he was now clearly and unambiguously the Irish head of state was celebrated by President Ó Ceallaigh by visits to the Holy See and France. A visit to meet King George in Buckingham Palace was also provisionally planned, but timetabling problems with the President's schedule prevented the meeting.
On balance, the weight of evidence would suggest that King George VI, as King of Ireland, remained on as Irish head of state until 1949, when the key international representional role previously performed by the King was vested instead in the President of Ireland under the Republic of Ireland Act, 1948.