I suppose a mini-Schengen for movement of people would be achievable, but the movement across the border of goods is a minefield. Every car would have to have its boot checked (trunk, yanks). Either there's full border controls or Ireland has to be allowed to be a special case. Can't see it somehow. The EU is scared of making special cases, which is why David Davis is finding that queen, country, Biggles books and empire are getting him nowhere. He's the last man on earth who we need to be on a learning curve, tragically for this country.