And the EU are completely right to protect the GFA
Suggest you look at Article 50 in more detail. Key points are:
Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements
GFA is not part of UK Constitution. This was established in the Miller case whereby supreme court ruled that UK does not need to obtain approval from Northern Island to leave the EU. Makes sense based on fact that; England, Scotland and Wales were not party to any referendums or votes for the GFA.3.
The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period
This recognizes that if agreement can not be made the member that has chosen to leave does so without a deal. This makes sense too as the leaving member could be trapped in the EU forever.
Substantive conditions for a withdrawal
Relevant international-law provisions cannot be applied in parallel to Article 50 TEU. Rather, the procedure and consequences of a withdrawal from the EU are now governed by EU law and no recourse to international law is possible
The GFA is not included in EU law and as the above states there is no recourse to international law if a member chooses to leave the EU.
Procedure
Ratification by Member States
Unlike the accession of new Member States to the EU, the withdrawal of a Member State does not require ratification by the remaining Member States – Article 50(1) TEU mentions (in a declaratory way) only the decision of the withdrawing state, in accordance with its constitutional requirements
The above allows UK to leave the EU even if other member states do not like the decision.
The role of the Court of Justice
Again unlike accession treaties, the withdrawal agreement is not primary EU law, since it is concluded between the EU and the withdrawing state and not between the latter and the rest of the Member States
This requires to European Council, as opposed to individual members, to negotiate the Withdrawal Agreement.
If EU is allowing ROI to take the lead then they are in violation of Article 50.