We can split hairs about what is and is not democratic until the end of time, but the debate takes place in a historical vacuum: the creation of Northern Ireland was a response to a very specific set of circumstances, and I don't think yak-shaving about democratic niceties casts any light.
It is moot anyway, because there is no prospect of changing Northern Ireland's status without democratic sanction. Which vote of course takes place at the pleasure of His Majesty's Secretary of State for Northern Ireland.
Can you point to a vote by the Irish people in favour of independence?
I'm pretty sure you can't given that no such vote has ever taken place.
The IRA derived their 'legitimacy' from it.
And to be quite frank, you are probably thinking of the Provisional IRA when you put legitimacy in quotes, where as this was the original IRA, who basically have nothing in common with the Provos (as they were called when I were a lad) except the name.