It is not fringe thinking to not want to be in Schengen. Maybe it is different on the continent where nobody has that opt-out, but thinking you know better and disrupting the trade of a member who negotiated what was supposed to be essentially a final arrangement in good faith (Ireland) will not be well-received by that EU member. It would raise serious questions for Denmark, Poland, and Ireland's (surprise) other opt-outs too, all negotiated with the European Union in good faith. The Schengen opt-out might not be your thing, but it adds a little more assurance for a great number of people on the fringes of Europe, and most of those people are near the centre of the political spectrum I would venture to guess. Not your enemy. Schengen is not part of the four principles in regards to Ireland, which has a very, very old opt-out that should hopefully be respected. Everybody knows that as long as free movement for people entitled is continued, then everything is fine. It's not an opt-out that would be made anymore, but it was supposed to be entirely up to the UK and the Republic of Ireland, one of whom isn't leaving.
The best deal (though I'm not saying this is what will happen) will still entitle EU citizens to that free movement in exchange for single market and customs union access (obviously for the whole UK). It will just be in the CTA as it has been for a long time now. It'd be a good compromise that would make nobody happy.
If you someone has no legal right to free movement as the EU, the Republic of Ireland, and the UK have decided, then they just don't, period. What I'm talking about would not affect EU citizens any more than it already does. Everyone found a situation they could live with in 1998, partially thanks to the EU perhaps. It is not too much to ask to allow one of the earlier members (Ireland has another opt-out and initially rejected the Lisbon treaty too) to continue on with its old travel arrangement. As an early member it had a special choice and that choice is supposed to be respected permanently in all future negotiations. You may find it a bit strange, even stupid, but I really don't think it's too much to ask that you would respect the archipelago's opt-out.
It would cause a disruption to trade, plus we just saw this week that Schengen is risky (as extremists can take advantage of completely open borders) and Ireland has been with the UK in rarely talking about leaving the opt-out for Schengen. Last time this seriously came up the Irish Taoseach said, quite simply,
"no." I don't think that position has changed. It may be talked about at first, but I don't think the EU will seriously try to force Schengen on the UK (and thus the Republic of Ireland). Nobody wants to fuck over the Republic of Ireland and Northern Ireland in proceedings and it would be entirely on the EU if they essentially bypassed Ireland's permanent opt-out (or rather, the reason for that permanent opt-out), not the rest of the UK.
I have a strong feeling that things could work out if everyone would just compromise.
Again, one UK-wide deal that will make absolutely nobody happy (especially the SNP, the right-wing of the Tories, and UKIP, but genuinely, I mean
absolutely nobody) would really be best. It'd be "Norway minus" or "Switzerland plus-minus". It can work. The arrangement might even grow on people over time. I suspect a lot of people are content in those countries with the arrangement and I believe it can work.