Actually in 1972 the U.S. could do exactly that. The U.S. was just about 100% out of Vietnam (pulled around 400,000 combat troops out of country in 1971-72) many of theose troops were still completing their two year (in the case of the Marine Corps 3 year) enlistments. Not only could the U.S. drop a Corps in, it would be full of combat vets, including a lot of troops with tons of experience in COIN and hunting VC. The UDL and/or the Real IRA/Provos thought the SAS was hard core. The SAS was solid professional, minimum combat time with really strict ROE (which, BTW, is exactly what you WANT in something like The Troubles). The Americans would send a combination of mostly MP, but with enough Force Recon and probably 173rd Airborne to deal with things when necessary.
Also important to keep in mind that this is when Nixon was POTUS and the when the "Irish American/Irish Catholic" vote was one of the most potent voting blocks in American politics (as a measure, combine the influence that "evangelicals" have in today's GOP with the "college educated voter" in today's Democratic party, the Irish Catholic vote was, along with organized labor, the 800 pound gorrila in National Elections). A bit of video showing any sort of atrocity toward the Catholic minority (or frankly ANY sort of violence against Irish children, regardless of religion) on CBS Evening News with Walter Cronkite and/or on either of the other networks, and there will be 400+ members of Congress DEMANDING action purely in interest of saving their jobs.