"It is representational democracy like all major countries have."
Quite so - we have a parliamentary system. As far as I know there has only ever been one referendum put to the people of the UK as a whole and that was over membership of what is now the EU. There have been quite a few other referendums concerning constituent parts of the UK/GB or regional parts of England and in all of these referendums the vote was only given to the people directly affected in these various constituent parts of the state or regions of England. For example the two Scottish devolution referendums or the London Mayor referendum etc. That is a solid democratic principle. Scots would have simply not accepted that people outwith of Scotland should have the deciding vote on what happened within Scotland. The Scottish people chose their preferred option and that then had to be passed through Parliament for what was basically a rubber stamp. The same would be so for Northern Ireland - and I can't think why anyone should think they deserve less democracy than Scots, Welsh or Londoners got?
As far as I see it various opinion polls in GB have shown that the British people favour the idea that the people of Northern Ireland should have the right to choose and all the major parties back that idea. 90% or so of the people of the Irish Republic who voted agreed with that. And a majority of the people of Northern Ireland agree with that too. It wasn't broken down so that you could tell which religion voted which way - but I believe that the estimates are that the Catholic population of Northern Ireland voted the same way as the Republic with about 90% accepting the principle. The Protestant majority was still there but was much smaller with a substantial minority saying No. In other words the only people against it seem to be a small proportion of hard line Nationalists and a larger proportion of hard line Unionists. Both probably thinking that their opinion should over-ride the opinion of the people as a whole.