A lot of English (sic) people have very little understanding of the border between the Irish republic and part of Ulster (the 'six' counties).
Currently, with free traffic of people and goods both ways, the almost unpoliceable border is no problem.
Post-Brexit, it will be like having the Channel Tunnel with no customs posts at either end - unless collectively we spend billions on some sort of physical barrier (plus a permanent border-policing force).
For my part, I wouldn't mind the fact that any foreigner who fancied it could go to the Irish Republic and then just waltz across the border into the United Kingdom (for instance), but I think the White Van Man (and his pals) would be up in their tatooed arms over it...
BB