It's long confused me that the DUP were pro Brexit; any Brexit always seemed to me to raise the likelihood of a united Ireland coming sooner rather than later.
I didn't say they were smart.
With the caveat that my knowledge of partisan politics doesn't emanate from Ireland, it's highly likely that there's certain amount of believing your own hype here ... the DUP are raised on tales of how they are superior to those in the Republic, etc etc. Which could have led to a judgement that as soon as the Brexit crowbar was applied, the Republic would (as they've taught themselves over the years) fold and it would be like the good old days.
Instead, the Republic not only didn't fold, but turned up with 27 mates for moral support. Moreover. 27 mates who do have a dog in this fight and aren't going to give up on a friend in need. (Which because it's what the DUP would have done, is what they assume others would do).
Moreover, the dragging out of Brexit (which let's remember was not only not mentioned, but rubbished by Leave) has meant there has been time for the people on the ground to realise what is happening, what is likely to happen, and how best to deal with that.
I'm probably miles wide of the mark. But while on the one hand, the situation in Ireland is unique, on the other hand, it's a creation of humans. And human behaviour the world over, since time immemorial has been tiresomely predictable - especially in masses.