One of the thorny Brexit issues will be Northern Ireland (Ulster). This has a land border with Eire which will remain in the EU. Currently, there is free movement north and south across this border. What happens when the UK leaves the EU?
Most unionists do not want a united island of Ireland. Most in Sin Fein would like to see the island reunited. I, for one, do not see how this will be resolved. Maybe a political "fudge" with the north somehow independent? No, I think Theresa May, the UK PM, has a really tricky issue here.