As someone from Ireland, I appreciate the solidarity. But at the same time, I'm not sure it isn't still a bit misplaced. Note please that I am not a person arguing for a time-limited backstop. This is kind of the vibe I'm getting from your posts. I think you misunderstand the point.
The GFA is not an EU matter, though EU membership is something of an assumption for the parties. That's why it's so weird to hear you talk about the agreement. I wonder have you actually read it? It's not long. And SO many people who argue about it have never read the document.
As an example, customs checks are nowhere mentioned in the document. Neither specifically are rules about what the border should look like. The agreement obligated the UK to 'normalise' the border--which meant removing the military presence, the 'security installations'. To understand what it really means is just to look at what the UK did after signing, up to this present day.
What Ireland care about most is not a material breach of the agreement, as it relates to the border. What we care about are the effects of a border: re-kindling of violence and sectarian tension; harm to the economy and social relations of the border communities on both sides; and worst of all, changes to the 'status' of NI without their express consent. We don't want to see 20 years of peace and advancement rolled back, communities decimated, and political oppression return to NI.
This is what the UK does not understand about NI. It's not just a part of their 'kingdom' or just some territory of the UK. NI is entirely a special situation--having a foot in both Ireland and the UK. Having been a violently disputed territory since Partition began. The idea that NI could figure out for itself its own future is what brought peace.
Brexit is a threat to this, whether or not NI stays in the CU or not. The DUP does not speak for NI, they speak for but a fraction of it. The 'hard border' is a symbol of a change put upon the people of NI without their consent. In fact, against their consent. The DUP as it were are not protectors of the status quo created by the GFA. They didn't sign it in fact--and their their demand to cling onto Westminster (and to be important for once in the UK) comes at the expense of 65% of NI. Who are very unhappy about the direction the DUP are pushing NI.
That is, in summary, my way of saying that the backstop is about the status quo, and the status quo is built upon the GFA. That's all the backstop does--guarantee NI stays the way it is now. And the reason we don't want a timelimit on it is not so much Perfidious Albion. Rather it undermines the right of NI to self-determination.