The answer is clearly yes as far as the EU’s jurists are concerned. But could they be wrong?
Ostensibly, the sole and single purpose of the Irish backstop under the WA (Withdrawal Agreement) is to prevent a breach of the GFA in the event that an FTA (Free Trade Agreement) is not reached during the (Brexit) transition period under the WA.
If it were at all possible, under ANY circumstances, to have a hard external border between the EU and the non-EU country which the UK will become after Brexit, and yet NOT breach the GFA by so doing, we’d have heard about it by now*. The EU Commission is absolutely insistent that the backstop is the ONLY way to safeguard the GFA which, by the way, is an international treaty.
It follows that the hard border which must follow a No Deal departure from the EU by the UK on Oct. 31 (or thereafter) MUST breach the GFA. At least as far as EU jurists are concerned.
So what about the text of the GFA itself? Anyone can in fact read the full text of the GFA. It’s a bit long and boring, as one might expect, but not particularly difficult language.
I’m not a lawyer or an expert, but I failed to see many references at all to the question of the or a border. This is partly because the thing is predicated on the assumed non-existence of a physical border. It several times makes reference to “cross-border” and “all-island” matters but doesn't say anything about the nature of this border.
Regardless of the social, sectarian**, political, economic and other consequences of a hard physical border, I rather surprisingly came to the conclusion that pretty much all the provisions of the Agreement could be satisfied despite there being a hard physical border, as far as I could see.
I don’t believe that the arguments and conclusions of the EU’s jurists are available for public scrutiny. In posing this question I'm really not asking for speculation. It is clear that the GFA is predicated on the existence of an island where there are no physical borders. But on the subject of such borders it is silent.
Continuing the functioning of cross-border bodies would be more irksome with a border present, but "irksome" is not "breach". Equally it would not prevent a border poll (Reunification referendum) being held at some point. And so on...
Does anyone know where these EU jurists' written findings can be found and read? Or failing that does anyone know the gist of these findings, i.e. in what exactly this breach is said to consist, and precisely which provisions would/will be breached on October 31 with No Deal, or following failed Free Trade talks if there is a Withdrawal Agreement?
* There's a very long and mysterious answer here about whether the EU will in fact require a physical border in the event of No Deal. If it can be avoided that way, in that eventuality, why couldn't that solution equally be implemented in the event of failed FTA negotiations, instead of the Backstop? Mysteries, mysteries...
** By listing the problems here, and specifically including the word "sectarian", I am not in any way whatsoever underestimating these difficulties. It can perhaps be argued that in reality a physical border is now politically impossible for Republicans, north or south. But if that is the rationale for the Backstop, rather than a legal breach of the GFA, I think we need to know that.
addendum (added after phoog's first comment)
phoog in her/his comment claims that the purpose of the backstop was essentially to keep the border open. From googling I've only been able to find out so much. If someone can point the EU Commission's own reasoning this would be immensely helpful. Nothing better than the horse's mouth.
But if that is the case I find the stance of the EU, and particularly the Republic (of Ireland, aka Ireland), even more ironically tragic than at first thought. My understanding has always been that because the GFA is an international treaty the EU's jurists could not consider a breach of it because that would be contrary to international law. There was, faced with the intransigence of the UK govt about the backstop, always an irony that (in the event of No Deal) the very thing designed to prevent a breach would have the consequence of causing that breach.
If it is in fact the case that there would be no breach of the GFA if a hard border were to be installed there, and that the rationale is in fact to maintain peace and stability across the island, the irony then becomes even more stark, since then the ONLY thing preventing the continuation of peace and stability (in the event of No Deal) is the desire to have a guarantee of peace and stability. I.e. international treaty obligations play no part in the reasoning or decision-making.