I would argue that Ireland was not as unified as you're saying, and it's disingenuous to display it as such. If Ireland was so anti-British, why would the Constitution of 1937 pass by 56.5% and not something higher? If Ireland was as unified as you say, why was the constitution opposed by members of Fine Gael and the Labour Party? Because it wasn't a unified nation. Whilst most Irish establishments such as the Irish Independent celebrated it, alongside most Irishmen, The Irish Times for example criticised the constitution's assertion of a territorial claim on Northern Ireland, and the absence in its text of any reference to the British Commonwealth.
And why, if they were already completely sovereign, would the constitution make mention of national sovereignty, saying that it provides 'inalienable, indefeasible, and sovereign right' of the Irish people to self-determination (Article 1) and that Ireland itself is declared to be "sovereign, independent, [and] democratic" (Article 5).
Why were these highlighted as important parts of the constitution if Ireland was already, completely independent? Fact of the matter is, they weren't, de jure yes, like the rest of the dominions, but not de facto.
Two words: political theater. All of the treaties/agreements/disputes (on both sides) were political pantomime to secure votes at home. Some parties in Ireland wanted to pay lip service to dominion status, in an effort to secure trade with the UK for the economy's sake. Some parties in the UK wanted to pay lip service to dominion status to keep from having to face the national embarrassment that the vaunted British Empire was fracturing. Some parties in Ireland wanted to pander for votes at home by loudly railing against the 1922 treaties because it's easy to say "Ireland should be whole and united", if you know there isn't going to be a war over it for practical reasons. By playing the "autonomy/independence in stages" game, the British could save face and the Irish could consolidate their gains and build Ireland into a modern nation (Ireland was an impoverished 3rd world country in 1936). There were reasons to pretend various things on both sides, but don't have any illusions about this perceived Irish disunity: the Irish were united in disdain of the British. If the UK had gone to war in 1936 over the Rhineland, and tried to force Ireland to join in, the Irish would've refused to participate. If the UK had tried to use military force to get Irish compliance, the Irish would've fought back as a unified nation. As long as the UK stayed on its side of the border, the Irish would happily fight among themselves. If the British tried to reclaim their authority over the rest of the island, that infighting would cease for the duration of the conflict.
Nothing unifies people, any people, like a common, external enemy. And the British had been the enemy of the Irish for 700 years by that point. That's an animosity that takes a looooooong time to fade away.