I doubt many Brexiteers gave the Irish border much thought when they voted, it wasn't discussed very much

All can about that is that it was mentioned here by, for example, McGrath of Harlow on 2 June 2016 and yours truly on 9th July 2016 where we said we would be in the position we are now.

All dismissed out of hand becauae the border would not be a problwm, naturally.