Brexit Impact on Northern Ireland

The aspiration clause in the Constitution - Art 3.1 - itself explicitly envisages a future consent to reunification by a majority in both jurisdictions, so I don't think you can really argue that the referendum to adopt Art 3.1 constitutes the necessary consent. If it did, Art 3.1 would be differently worded. Besides, it's almost certain that reunification would required constitutional amendment - Art 3.1 itself would need rewriting or replacement, for one thing - so a referendum in the Republic would be needed.

I take your point about an white paper in advance of any NI border poll. That might be a very good idea, but the GFA doesn't require it. And it would have the disadvantage that the the white paper would have to be prepared at a time when the level of support in NI for reunification was unknown. For the reasons already pointed out, it might be desirable to do this after the border poll, rather than before.

The other question you have to ask is, who would prepare the white paper? You suggest that "the Irish government will produce a white paper", but the GFA says that the proposals for reunification must be agreed between Dublin and Westminster. I think that means that the white paper would have to be a joint IRL/UK production, or at the very least that the IRL government would have to get Westminster signoff on the white paper before issuing it. Either would give Westminster an effective veto over the reunification decision (by simply not agreeing to any White Paper) which is contrary to the whole point of the GFA.

I think what the IRL government could do is (if they thought it a good idea) is issue a white paper saying "if the border poll passes, this is the framework for unification that we will propose to Westminster". (And that is pretty much the status which the Scottish government's 2014 white paper had.) But there would be no guarantee that Westminster would accept it without alteration, or indeed that the IRL government might not themselves want to modify their proposals in the light of the border poll outcome, or developments after the border poll, or representations made to them by stakeholders.

At best, what NI voters would have before them when voting in a border poll would be a framework proposal from one side for the terms of reunification to be agreed with Westminster after the border poll.

And actually proceeding to reunification would almost certainly require a referendum in the Republic, conducted after terms for reunification had been agreed between Dublin and Westminster.

In that scenario, it would be all but impossible to refuse a parallel referendum in NI.

(And why would anyone want to refuse it? The democratic legitimacy of reunification would be pretty much cast-iron if approved twice by the people of NI, once in principle and a second time in detail. Conversely, if NI only gets a referendum on the in-principle question but IRL gets a referendum on the detailed, legally effective terms of reunification, that puts a major question-mark over legitimacy, which the IRL government would very much not want. Plus "unequal" referendums like this would be constitutionally questionable, given the terms of Art 3.1.)

GFA says that the reunification proposals are to be agreed between IRL and UK governments. Are you repudiating the GFA, Francie? 😉

There's no problem with a second referendum that can reverse the first - we've done that a couple of times in IRL already. It's not remotely comparable to a supermajority requirement. Plus, the GFA explicitly contemplates that there can be more than one border poll, and it's absurd to argue that a border poll is final if Francie likes the outcome but not if he doesn't.

Only Brexiters think that the purpose of a referendum is to put some question beyond democratic review. Don't be like a Brexiter, and don't aspire to a Brexity united Irish Republic.

As I see it, it'll be part of point 6 - proposals for reunification will include changes to be made to the Irish constitution, which will require a referendum. Or, even if there is by some miracle to be no amendment to the Irish constitution, the proposals will include a referendum in IRL anyway, to satisfy Art 3.1 of the Constitution.