“Can I ask NJ the question posed a while ago; it is the UK who want to stop free movement of people. How can you do that with an open border in Ireland? No checks, no customs posts.
The Republic of Ireland is a full member of the EU.”
Course you can.
There has been no effective border between the UK and Ireland for a hundred years – long before the EU was even thought of. That was a decision taken between the two parties. The fact that the EU allows freedom of movement across all its member nations is not relevant to the question of a borderless arrangement between Ireland and the UK.
Furthermore, there is far more to free movement, EU style, than simply movement. It also bestows the right to settle, work and receive benefits. If the UK leaves the EU (properly) it will be able to determine who settles, works and draws benefits here. So whilst citizens from other EU countries may cross the border from Ireland (though I doubt there will be many) they will not have right of abode, employment and benefits. That will be a matter for the UK alone.