I do think it's important that incorrect information is called out (e.g. the stuff about how it is undemocratic to allow the GFA impact on brexit as more people voted for brexit than the GFA)
Lets tackle this on the basis of Law;
Article 50 states:
Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements
Good Friday Agreement is not part of UK's Constitution. This was established in the Miller case whereby the supreme court ruled that UK does not need to obtain approval from Northern Island to leave the EU. This makes sense based on fact that; England, Scotland and Wales were not party to any referendums or votes for the Good Friday Agreement.
Article 50 also states:
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 cannot be made then the member that has chosen to leave the EU does so without a deal. This makes sense too as the member who wants to leave could be trapped in the EU forever otherwise.
Briefing notes from the European Parliament February 2016 read:
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
This recognizes that EU law is not the law of the World. Rightly so as EU is 28 Countries out of a total of 195 in the World.
Also recognizes that when all 28 members passed Article 50 in 2009 that they had performed their own due diligence and researched how Article 50 could affect them as individuals in relation to any arrangements they have with non EU Countries or any agreements made prior to 2009 when Article 50 was passed.
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
This recognizes that all EU members are free to leave the EU without the consent of other members.
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 recognizes that if a member chooses to leave the EU then it deals with those that head the EU as opposed to individual members. Essentially the European Council which is made up of the Prime Ministers of each of the EU members.
Summary
Based on the provisions of Article 50 the Good Friday Agreement has no influence whatsoever whether or not either Ireland or the UK can choose to leave the EU.
The Good Friday Agreement, while given the status of an international treaty in 1998, was not incorporated into EU Law when Article 50 was passed in 2009 by the European Council in 2009. At that time UK Prime Minister was Tony Blair and Ireland Taoiseach was Brian Cowen.
EU law in not international law. This is recognized by the European Parliament in that disputes over Article 50 can only be dealt with by the Court of Justice (ECJ) and no referral can be made to international law.
Next point;
or that the UK government has no choice but to no deal and that they are the victims in this narrative, being bullied by the big, bad EU
Lets tackle the legal position:
Article 50 permits a member to leave the EU without a deal.
Negotiations on the Withdrawal Agreement take place in accordance with Article 218(3). This Article describes the procedure for negotiations, but nothing else.
The negotiating parties are free to agree whatever deal they wish. If agreement is not made then the member that applied to leave the EU does so without a deal and all laws and treaties lapse between the EU and the member who has left.
Certainly nothing in Article 218 that states the negotiators appointed by the European Council must only offer terms that take into account any agreements made between member states for their own purposes prior to Article 50 being passed.
So while a mutual deal can be made between EU and UK that takes into account the Good Friday Agreement there is nothing in law that states such a deal is obligatory.