Of course it is silly but he would be a symbol, or the most obvious recognisable symbol of the British Government which had forced the re-enforcement of the border on the island of Ireland.
Entirely innocent of course but then a lot of your casualties in the Troubles were.
Loads of nutters still mooching around, on both sides. Not sure they will be too arsed about fighting each other so your boys will be next in line. Nothing may ever happen, hopefully not, but the threat will be back for the first time in over 20 years or whatever.
Sad scenes.
I seriously doubt there is the political will or capability to support any sort of large-scale return to terrorism. There are no serious numbers interested in starting the Troubles up again. An entire generation has grown up in peace - you might get a couple of nutter dinosaurs blowing up fertiliser bombs or shooting at a customs official, but the chances of it going back to anything like the bad times is pretty much zero.
First, you prevent them coming by making it not worth their while - no rights to employment, housing, welfare or health provision, etc, etc (as we'll be free to do when not in the EU). They'll soon fůck off back to the Republic - and if they don't, you physically deport them.
Also, you can impose stringent identity checks on such migrants to tell you who is or isn't in your country and severely limit their rights. They can also be prevented from travelling from NI to the mainland (as we used to with certain people during the Troubles).