Why should they? It's the UK which has decided that it wants to Leave, so it's up to the UK to try to square the circle
Article 50 allows any member state to leave the EU. Remember that Article 50 was developed in 2009 and agreed between all EU members at the time, ROI included and made no reference was made to the GFA that was developed in 1998. A huge oversight by the EU
Article 50 reads:
2. A Member State which decides to withdraw must notify the European Council of its intention. In light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State..........
3. The Treaties shall cease to apply to the State in question from the date of the withdrawal agreement or failing that two years after the notification in paragraph 2. unless both the Member State and European Council agree to extend this period
If the negotiations fail then UK leaves without an agreement and all Treaties between EU and UK cease to exist and in terms of borders UK becomes a third country.
Under EU law there is no freedom of movement of people or goods between EU and non EU Countries. Therefore each member state of the EU has a responsibility to control passage of persons and goods from outside EU. In this case ROI, as a member of the EU would have install a border between NI and ROI as NI is part of UK which would have third country status.
Article 50 does not go on to say that the member who has left shall remain responsible for the EU's future actions and ability to comply with it's own laws and regulations.
EU laws on border control came into effect long before GFA was developed and EU are not a party to the GFA. Hence GFA does not overrule the EU Laws.
It would be morally wrong to say "tough luck" to those whose daily lives rely on easy passage across the invisible border between NI and ROI, but when it comes to battles and legal arguments moral considerations are usually ignored. So target should be to avoid such battles.