No one doubts there will be a border between the republic of Ireland and Northern Ireland: in fact there is already a border.
The question is what, if any, new infrastructure is installed at the border and on which side(s) of the border. No one (except maybe Pascal Lamy and friends?) wants any infrastructure installed at the border, so it would be the height of stupidity to install such.
And having seen the performance of May and co. you think building such infrastructure being stupid is actually going to stop it happening?