It is up to them to do so in a way that complies with their obligations under the GFA
No such statement in GFA that if either North or South left the EU it was sole responsibility of the part that left EU to ensure GFA maintenance.
Parity of rights, mutual consent and self determination between North and South seem to be the key elements of the GFA.
GFA makes several references to EU such as:
Coordination between the Assembly and UK of EU issues. Obviously when UK leaves the EU this requirement is no longer applicable to UK, but ROI as an EU member will have to coordinate both GFA and EU requirements.
GFA also makes it the responsibility of North and South to resolve disagreements including any in relation to the EU. This too is inoperable as NI will not be in the EU when UK leaves the EU. So it then becomes South to resolve any disagreement arising from EU.
Reference is made in GFA to implementation of EU policies by North and South. Easy when both are in the EU. This is the ROI conundrum in that it has to find a way to balance EU policy and GFA. North does not have that issue as they will cease to be an EU member. Even Varadkar has acknowledged that if no deal occurs there will have to be checks on goods entering ROI from NI
Strand Three of the GFA describes the role of the BIC (British Irish Council). They are to meet regularly to exchange information that is of mutual interests including EU issues.
This is the error. The BIC failed to take into account Article 50 that entitles any of the EU members to leave the EU by making an amendment to the GFA to cover such eventuality.
Many references made to EU; policies, programmes and issues within GFA, but ignored when Article 50 developed and agreed by ROI and UK.
Astonishing