Sure. If your employment contract prevents you from taking other work, you may be fired if you break this. You may also have signed up to binding arbitration by some weird court. But normally the 3rd party (your weekend employer, who did not sign) cannot be penalized.
That's what points out how much sovereignty has been given up by Ireland. It has signed up something much more than a trade deal. A foreign court can not just strike down laws it passes, but re-write them retrospectively a decade after the fact.
No law has been sturck down. No law has been rewritten. No law has been applied retroactively.
A deal was found to be illegal, based on the laws at the time, so was anulled (as any illegal deal would be).
This was appealed, and the court found that the facts of the case had been misinterperated (ie they're arguing about what counts as state aid, not whether state aid is illegal).
And not just Ireland's.
And that's what I'm saying. If your legal system is united to this degree into a larger framework, then that new thing is the locus of sovereignty. It is much more than a trade deal. (That doesn't mean it's wrong, or a good idea or a bad idea. But it's a big change.)
Ireland signed on to the EU and several trade agreements before that. Until it quits these it is subject to their rules.
> If you negotiate a deal with the highest authority in Ireland, and they sign off on it, are you done?
Any contract you sign in any country can be invalidated by a court.