I’m not sure what your point is? We all know the British and US armies have done terrible things in their time. Does the fact Bloody Sunday happened suddenly make Bloody Friday acceptable?
The proof of the pudding is in the eating. How many MPs did Sinn Fein get during the Troubles? They got Bobby Sands and Owen Carron (technically not Sinn Fein MPs) from 1981 to 1983 and Gerry Adams from 1983 to 1992 and then again from 1997 on. Martin McGuinness got in at the very tail end of the Troubles in 1997. By 1997 it was very clear Sinn Fein was looking for a way out of the Troubles. In the south they got no TDs. Throughout the entirety of the Troubles, the SDLP got the majority of Catholic support. The vast majority of support for the Provos was of a sort of reflexive sneaking regarder type because they were supposedly fighting the British and the British fought dirty. But the Provos fought very dirty too. How much support was there for the concept of eternal violence until a united Ireland is achieved? I would say almost none. That can be demonstrated by the widespread celebrations when the ceasefires happened. People wanted peace. The Provos campaign had failed, there was no united Ireland. But still people were celebrating. Why? Because it promised an end to violence. Much of the Catholic community itself lived in fear of the Provos, the Provos murdered 338 Catholics, a larger number than any other single group.
Northern Ireland was and is internationally recognised as part of the United Kingdom. Ukraine is internationally recognised as Ukraine, including Donbas and Crimea. These places voted for Ukrainian independence. They are part of Ukraine.
International law matters.
In many European states there have been and are pockets of ethnic groups In “other states”, ie the Germans in Czechoslovakia in 1939. Was there any real difference between that and people who identified as Irish in NI? There is no state of nature that says a 32 county Irish state should exist or has any right to exist. The notion that it has is essentially the same notion that says a Greater Germany including parts of what is now Poland or Lithuania has a right to exist, or a Greater Serbia including Kosovo and parts of Bosnia or Croatia has a right to exist, or a Greater Russia including the former USSR countries has a right to exist. But none of these things have a right to exist.
Any Ukrainians identifying as Russians are in the same boat as German identifiers in the Sudetenland in 1939 or people who identify as Irish in the North.
The Russian invasion of Ukraine is the equivalent of Britain invading the modern day nation state of Ireland - Ireland as in the 26 counties. If that happened, obviously it would be condemned internationally, and Ireland would expect outside help to fight it.
No nation state exists in nature. They are all artificial creations. International law is what matters. It should not be tolerable for one state to invade another without being a last resort as it was in World War II (and I absolutely include the US invasion of Iraq in that).
In NI, the Provos did not have near enough support to win, much like, say, the Tamil Tigers in Sri Lanka did not. There’s no point moaning about “arbitrarily drawn lines”. These lines represent international law. Within those lines, the Provos had miniscule support. They could never win and it took them 27 long years to cop that.