I agree, Ireland being in the Allies without getting back NI in exchange is very ahistorical.
The Republic of Ireland can't get back what it never had. You would have to go back hundreds of years since Northern Ireland has not been part of UK. The majority of the six counties have always made it clear that they wanted to remain within the UK, and in 1913-14 threatened a civil war over discussions about Home Rule. The sensible parties in the Republic wouldn't have wanted to take a million armed protestants into a small and poor country, with a small army that could never have suppressed any rebellion. Though they wouldn't have publicly admitted that. But that was the basis of them agreeing to the likely partition. That partition occurred because the majority of members of the Northern Ireland Parliament voted to remain the UK in 1922.
Although the Republic never joined the Allies, for political reasons, and was officially neutral, nevertheless it:
- agreed access to an air base and overflying their air space and location of radar/radio stations for hunting subs in the Atlantic
- provided intelligence on German spies
- allowed the British Army to continue to recruit men from the Republic, and for them to travel freely between the two countries. There was a long history of certain regiments recruiting in Ireland. 50,000 Irishmen served in WW2, not counting those from Ulster. And men from Ireland continued to emigrate freely to the UK to work in the factories and mines. My guess is up to 50% of the men working on the Tyne and in Glasgow building ships for the Royal Navy were from Ireland.
Whatever fanciful notions sections of the IRA might have had, the fact is there were probably more Irish living and working in the UK by the 1930s than there were in the Republic. Of my grandfather's generation of Irish Catholics, only one (the oldest son who inherited the family farm) remained in Ireland by the 1920s. A few emigrated to the US, most to the UK.
It's difficult to imagine there was any strong support in Ireland for German fascism to invade the UK with the idea that the north might then become part of Ireland. Most knew the effect on their own families, and could hardly imagine that Ireland wouldn't also be invaded. Despite their officially neutral position, the Irish government secretly planned with the British Army how to resist a German invasion in 1940-41.
