Northern Ireland 2125?

To be fair, you're only British because you identify as British; if you didn't, you wouldn't be British. So this is maybe a distinction without a difference.

Plus, identification is the language used in the GFA, the international instrument that guarantees your rights. It affirms your right (a) to identify as British (or Irish, or both), and (b) to be accepted as British (or Irish, or both, in accordance with your identification) by both governments.

So, yeah, identification as British is the fundamental thing here.

Let me fix that for you:

" . . . the very clear implication being that this is what could happen if you try to implement a Brexit that requires the reimposition of border controls in Ireland".

Somebody saying that is not trying to screw you; he is trying to protect you from your your so-called "friends" in Britain who are completely indifferent to your safety and welfare and would sell you in a heartbeat, and from their useful idiot lickspittles in Dundela Avenue.

I get that, as a unionist, you would rather be protected by the British. But, to be blunt, beggars can't be choosers; you should take whatever protection you can get (and postpone to another day the stressful task of reflecting on the implications of an Irish government caring more about you than a British government does). And you certainly can't blame Varadkar for the failure of the British to care about you.

so you are telling us francie is not Irish, he’s just identifying as such.

No, the opposite. Francie is Irish and he identifies as Irish. These are not inconsistent; in fact they usually go together. In NI in particular, how you identify is foundational, because the GFA makes it so.

you should also check the legal position. Everyone born in ni is British until they officially denounce their British citizenship - which a tiny number have done. They are also free to choose Irish citizenship

(Almost) everyone born in NI is a British citizen unless and until they renounce British citizenship. But the GFA distinguishes between being British (or Irish) and holding British (or Irish) citizenship; they are not the same thing, and you can do one without the other.

It works like this:

1. It is "the birthright of all the people of Northern Ireland" to identify themselves as British or Irish or both.
2. It is also their birthright to have their identification accepted.
3. Because they have those rights, the British and Irish governments both confirm that they can hold both British and Irish citizenship.

So, it's not the case that, for the people of Northern Ireland, having British citizenship make them British. It works the other way around; they have a right to identify as British and that identity must be accepted and, because their British identity is accepted, they have a right to British citizenship.

The point is, it all starts with identification. If you identify as British, then all the rest follows. (And similarly if you identify as Irish.)

(UK law confers British citizenship on most people born in NI by default, regardless of how they identify. That goes further than the GFA requires, obviously, which only confers a right to British citizenship in connection with identifying as British. But it's not inconsistent with the GFA; if you give everybody British citizenship by default, then obviously for those who have a right to British citizenship under the GFA, their right is being vindicated; they have British citizenship.)