I asked a good friend in the Dept. of Finance about that, and the answer was - no, they won't. Checkpoints targeting dozens of drivers is one thing, responding to a tip-off is another.
Too much hassle and not enough return (a few hundred € in taxes - IF the owner doesn't decide the car's not worth the bother).
I'm looking to buy a car and except for my excessive sense of patriotism, I'd only be too glad to pick up a cheap number in NI or GB. So I'll just suck it up and scowl at people less patriotic (or stupid) than I.