Irish politics are complicated but the gist of the reason for the Irish civil war was that Eamon DeValera knew up front there was no way the Belts would ever agree to a treaty that the Irish people would be happy with. He didn’t want to be the one to eventually take the blame so he sent Michael Collins to negotiate a treaty.
The best deal Collins could get was An Irish free state less the 6 counties in the North which are Often referred to as the 6 sorrowful countries. The Brits needed a place for the protestants to run to.
Eamon DeValera refused to accept or sign the treaty and he and Collins fell out. At that point Michael Collins, only 32 years old was running the whole country. He was wildly popular and engaged to be married to a beautiful girl, Kitty Kiernan.
Eamon Devalera co-opted the IRA which Collins had resurrected and turned them against Collins. The civil war that resulted from all this was in full swing when supposedly dealers sent word to Collins that he wanted a meet up with him down in Cork county near Skibereen which was Michael Collins home town. Collins was warned not to go but he’d had a very close relationship with Dealers for a long time so he went in an armoured car and escort and spent the night drinking in a bar in Skibereen waiting for word to meet but Devalera never showed up. The next morning Collins and his people headed back to Dublin and after they had traveled a short distance the IRA ambushed them at a place called Sam’s cross. The whole country went into mourning. Devalera is reported to have lamented later that he would never have any kind of recognition or a legacy because everyone blamed him for the killing of Mick Collins.
DeValera was a real piece of work. Even went to the German Embassy to sign the condolence book after Hitler offed himself.