I don’t see that idea as being anything but a token and utterly meaningless policy.What would that consist of or what would it’s purpose be?
Genuine question. If you are talking of all Ireland and Scotland, why? And what purpose outside of the EU?
If all were within the EU, then why, again?
As you’ve rightly said, if Scotland were to leave the UK and if it then entered the EU, both Ireland and Scotland would be adhering to the FOUR freedoms and would essentially be as close as two separate nations could possibly be anyway, as they would with the rest of the EU.
Now, as I’m typing this, there could be a defence arrangement where they share defence, as, let’s face it, Scotland’s army, navy and Air Force would be pretty nonexistent in the event of them parting company with England. But it seems the EU might go down a combined route anyway.
So yeah, my verdict is that it’s a token idea but pretty meaningless.
