Because, assuming the vote yes, that’s just the way you leave the UK:
The agreement was signed in London on 6 December 1921, by representatives of the British government (which included Prime Minister David Lloyd George, who was head of the British delegates) and Irish representatives including Michael Collins and Arthur Griffith. The Irish representatives regarded themselves as having plenipotentiary status (negotiators empowered to sign a treaty without reference back to their superiors) acting on behalf of the Irish Republic though this was never accepted by the British government. As required by its terms, the agreement was ratified by the members elected to sit in the House of Commons of Southern Ireland and the British Parliament. In that sense it could be regarded as a treaty but it was not between two states. Dáil Éireann for the de facto Irish Republic also ratified the treaty. Though the treaty was narrowly ratified, the split led to the Irish Civil War, which was ultimately won by the pro-treaty side.
The Irish Free State as contemplated by the treaty came into existence when its constitution became law on 6 December 1922 by a royal proclamation giving the force of law to the Irish Free State Constitution Act 1922.
Makes sense to me anyway.