>> Nobody at the time foresaw that this might ever change, so <<
That’s absurd! The Maastricht treaty barely survived the House of Commons just a couple years earlier, only after securing an opt-out from adopting the Euro. EU membership was ALWAYS extremely unpopular in the UK. If the Irish signatories didn’t realize that, they were the biggest fools ever. If they knew that, and thought they could force the UK to stay in the EU by signing the treaty, then damn every last one of them to Hell.
The last thought in the minds of the Irish parties (north and south) negotiating the GFA was any possible consequences for the UK’s EU membership. The fact of EU membership was generally recognised as a lubricant in the process: but implications beyond Ireland were not the business of the negotiations. They were complicated and delicate enough without having to introduce extra-Irish/UK considerations.