Go Safe van parked on side of motorway where the sign says Garda Cars Only

Even assuming you are right and they can’t park there, do you think their mistake or law breaking would nullify any speeding tickets they issued at that time?

I don’t think those signs have any legal bearing anyway. Maybe they’re mentioned in the Road Traffic Acts somewhere but the lack of an Irish translation suggests they’re only advisory.

This is a complex and somewhat nuanced area. Evidence obtained in "deliberate and conscious breach" of someone's constitutional rights is not admissible in court against that person, except in very exceptional circumstances. So, the guards detain you in circumstances where they have no power of arrest; they beat the living snot out of you; they interrogate you brutally; you make admissions that are, um, contrary to your own interest. Those admissions cannot be given in evidence against you.

But that's clearly not this case. Here, the suggested illegality - parking in a place where they are not permitted to park - is not a breach of anyone's constitutional rights. Where evidence is gathered in a way that involves illegality but no breach of constitutional rights, it's up to the judge at the trial to decide whether to admit it, having regard to all the surrounding circumstances — whether the breach of law was inadvertant; whether it was minor or more significant; whether it bears on the nature or quality of the evidence gathered (if they had parked just down the road where parking was lawful, would the evidence they gathered have been any different?); everything that might be relevant.

I'm inclined to think that where the illegality complained of is a violation of parking regulations, there's a good chance that evidence would be admitted. But we can't be sure without an actual case with actual facts.