Hang on, I thought we were jumping up and down with excitement about being in control of our own borders.... apart from that one?
Wont all the nasty foreigners just land in Dublin and then enter the UK in the North before slowly making their way to your neighbourhood, and mine?
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 fùck 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).