You just gonna ignore the fact that your present government is based off of the "half foreign" Union, in which immigrants made up half of the population and wiped the majority-local Confederacy off the map thanks to having an enormous supply of experienced manpower?
They wouldn't be under control of the fucking Queen, they would be placed under "Joint Command" like they are elsewhere and perform the roles they are already supposed to. That's assuming they even bother to, they could just openly invite the hundreds of millions of Latin Americans to the USA, promising them citizenship and enormous pay increases. They have done this before to no ill effect to their legitimacy - they could even push propaganda that America was built by foreign fighters who fight for the "idea"
And the IRA types, the heavy hitters, rarely wanted a fully united Ireland and were content with running water and electricity. The living conditions of Catholics prior to the Troubles had declined to the point that they had severe malnutrition and archaic diseases, entirely due to the policies of the Northern Irish government at the time. Since their kids can now go to school, most of the heavy hitters held up the ceasefire, leaving behind the useless ideological dregs who had no tactical mind to them. It takes a lot to motivate competent, decent people to violence - this is because ideologues who whined about wanting a 32 county Republic were just mouthy retards while people who saw the value in their own lives of having a say in local governance (in this case, the police refusing to prosecute crimes against Catholics, for example) were far more competent. The UK itself only had to provide "equality" for Catholics, which it took them thirty years of war to bother doing. It's hard to believe that it all came down to something so petty, but it turned out most of the IRA were perfectly content with that.
The modern remnants are more like Antifa-tier retards, whereas the older Provos were oftentimes married men with children who had a thorough understanding of the political situation, a thorough understanding of local politics and had legitimacy as established figures in their communities.
In a civil war situation, these are the types who would be of great importance in the USA. Nobody gives a fuck about a fat LARPer who hides in the woods, but a Civil Engineer in New York who has the opportunity and clearance to deliver an oil-barrel full of thermite to the top of a skyscraper is a far different problem. Faggots shooting at deer and whining about taxes vs a Doctor who can quietly fix up wounded insurgents without alerting authorities via the hospital (how'd you get these gunshot wounds, Cletus?) - these retards who want to hide in the woods and play with sticks are just as much your enemy as any deradicalisation shill. They want you to abandon your cities, abandon any hope of harming their political stature, and go dress up as a deer and hop around the forest like a big gay furry.
Edit: an oil barrel of thermite is incredibly heavy, so probably not going to happen, but the point stands. Competent people are usually comfortable in their present situations, until they are not. All the state has to do is make sure that the people who can be a problem will not be inclined to do so. This is why they have no problem shitting on you, you can't and won't do shit that will actually hurt them.