A Wikipedia discovery (with the usual caveats about Wikipedia):

The bill to declare Ireland a republic was introduced in 1948 by the new Taoiseach, John A. Costello of the Fine Gael party. Costello made the announcement that the bill was to be introduced when he was in Ottawa, during an official visit to Canada. It has been suggested that it was a spur of the moment reaction to offence caused by the Governor-General of Canada, Harold Alexander, 1st Earl Alexander of Tunis who was of Northern Irish descent and who allegedly placed symbols of Northern Ireland, notably a replica of the famous Roaring Meg cannon used in the Siege of Derry, before an affronted Costello at a state dinner. What is certain is that the prior arrangement whereby toasts to the King (symbolising Canada) and the President (representing Ireland) were to be proposed, was broken. Only a toast to the King was proposed, to the fury of the Irish delegation. Shortly afterwards Costello announced the plan to declare the republic.

So my own country had a role in the creation of the Republic of Ireland! But wait, wasn’t Eire, created with a new constitution in 1937 by Eamon de Valera, already a republic? What did Costello think he was doing – and why was Fine Gael doing it? As you recall, De Valera led the anti-treaty forces in the Civil War 1922-23. By rights he should have gone the way of Michael Collins, but survived to found his own party, Fianna Fail. He realized that Ireland could not realistically turn the clock back to 1916, as Sinn Fein wanted to do, but he was still staunchly republican and when FF was elected in 1932 he took advantage of the Statute of Westminster (1931 – it gave legal equality to the dominions) and promptly set about revising the constitution of the Irish Free State to make Ireland more independent, an effort that culminated in the new constitution of 1937. FF and De Valera remained in power until 1948, when a coalition headed by Fine Gael came to power – and although FG were the descendants of pro-treaty side of the Civil War, apparently they felt obliged to out-republic De Valera with the Republic of Ireland Act of 1948!

If the story is true, Alexander of Tunis must have really annoyed them.