I agree that it is unlikely that Collins could have gotten all 32 counties into the Irish Free State. The six counties of Northern Ireland included 2 counties that were majority nationalist, so it was obvious the six counties were designated to keep as much territory as possible out of the Irish Free State while still keeping Unionists a majority (which explains why all 9 counties of Ulster were not included). However, for the sake of argument, let's explore.
1) I am assuming there is still an oath of loyalty to the British Crown and there is no republic. De Valera and others will still complain. That Collins managed to include all of Ireland is a major gain over OTL, but De Valera's people won't know this is a major coup. I still see a schism within Sinn Fein and probably a civil war.
2) The Unionists will likely wage their own war and not accept Dublin as the legitimate government. Taxes will not be paid, representatives will not be sent, and paramilitary organizations will be formed. However, I don't think there will be any conflict until Dublin attempts to enforce its rule which could be a while as long De Valera's faction is fighting. In other words, I don't see the Unionists invading and attacking nationalist areas although there may be some fighting in Protestant dominant areas to prevent the Nationalists from taking power. So there may be some initial fighting local to the six counties until a stable "border" is created.
3) Some areas of the six counties will be controlled by Dublin. County Fermanagh, County Tyrone, South Londonderry, South Armagh and South Down, and the City of Derry are largely nationalist. They will be loyal to Dublin. The Unionist controlled area is Belfast and the northeastern portion. In addition, Dublin also legally controls the territorial waters around Belfast and all of Ireland, but it will take time for any Irish Navy to assert actual control.
4) There is also likely to be an extended period of negotiation with the Unionists in an attempt to convince them to cooperate in exchange for specific pledges to be entered into law or the constitution.
5) The long term prognosis for Unionist defiance is not good. There are a lot of economic reasons for the Unionists to begin working with Dublin. The Protestants are the economic elite and they have a lot to lose. This is not the same as the Catholics during the Troubles who were economically marginal and actively discriminated against. They had much less to lose by their protests or violents than the Unionists now.
6) I think the government in Dublin would be content to let the Unionists to wither on the vine until they decide to cooperate with Dublin in return for major concessions. Different Irish leaders will be more prone to offer concenssions than others. If Collins is alive, I can see him granting the vast majority of Unionist demands. De Valera would likely be more obstinate.
7) After some point of time, Dublin will eventually resort to force to enforce its control over Belfast and other areas if negotiations fail. Given enough time to prepare, Dublin will win the conventional war. Casualties will be high in storming certain urban areas of Belfast, and the major areas of the city may be destroyed.
8) Assuming Dublin still grants significant concessions to the Protestants, and assuming that the Irish Navy can prevent arms smuggling from offshore into Belfast, long term guerilla warfare or terrorism is unlikely although the rest of the 1920s and 1930s may see sporadic attacks.
9) In World War II, there is a significant chance that the Irish government agrees to enter the war. After the war, chances of the Unionists resorting to terrorism is probably zero.