It seems that in both cases a long domination by a more powerful neighbor has left its imprint on a significant part of the country, and many of the people in this part of the country feel a loyalty to that dominant country.

So, why is it ok to split Ireland but not the Ukraine?

Ireland is in two parts today because the revolution pitched against direct colonial rule from England, started in the last days of Great War, succeeded sufficiently that after several years of fighting England offered a settlement which preserved the Ulster enclave while recognizing an Irish Republic in the rest of the island, and the leadership of the revolutionary forces felt that this was the best deal they could get at the time, and that they had better take it. Significant elements of Ireland's revolutionary militants did not accept the deal, and continued a sporadic guerrilla campaign, at times aimed against the Irish Republic as well as against England. The drive to assert Irish sovereignty throughout the Ireland was revived strongly by the outbreaks in Belfast in the seventies, and though it failed to expel England from the island, it did alter constitutional arrangements appreciably in the small portion still associated with England. There are still factions which feel this has not gone far enough, and remain dissatisfied with the arrangement....

The biggy is status quo. The borders were there and okay for 23 years.

In fact, the borders were still fine according to a majority of the population in the area.

In both, some few wanted to fight and kill to make the borders change. In each case, it was a relatively small minority that wanted the change. Britain supported a majority and status quo against a minority that was really okay with killing Protestants, British, and even Irish that were in the way.

Russia supports a minority that is really okay with killing non-Russian Orthodox, Ukrainians, and even ethnic Russians that get in the way.

If you want to look further back in Ireland you can find things that were seriously wrong by current standards. Those, however, are so common in history--not just W. European history, mind you--that trying to make a federal case out of that specific mess and call for reversing the effects of colonialization is a tough call. And, if you were to do that, you'd be demanding ethnic cleansing in N. Ireland. If there, why not elsewhere? For example, the Donbas. Brittany. Or even demanding that Moscovite Russians leave because that area was populated by Balts or that the Swedes be driven out of Finland.

We look for a reason to contemplate our navel and focus on ourselves while calling ourselves citizens of the world and supporters of human rights everywhere. Why should we trouble our beautiful minds about such things. We managed to ignore the USSR for decades as it killed millions. We didn't want to know about Hitler's Germany and turned our backs on refugees from there. China had repressions that many wanted to argue weren't actually repressions, but that was a harder sell. After Vietnam went entirely N. Vietnamese there was a mass exodus, but I knew former "boat people" who had to argue with people in solidarity with the "people of North Vietnam against oppression" that claimed all the boat people were CIA spies and collaborators and there weren't re-education camps. It's hard to feel good about yourself when those you support really are the reason for a lot of blood spilled, so it's psychologically necessary to cleanse the record. It's called confirmation bias.