At the moment the border between Northern Ireland and the Republic of Ireland is virtually invisible. If you land at Dublin Airport and drive north to Belfast (in Northern Ireland) you will cross the border with the only thing you notice is that the signs start using miles rather than kilometers.
Used to be that way between the US and Canada.
9-11 changed it and technology made it easy to continue as before.
NEXUS
https://www.cbp.gov/travel/trusted-traveler-programs/nexus
FAST
https://www.cbp.gov/travel/trusted-traveler-programs/fast
The UK Remainers do not have a leg to stand on in opposing Brexit.