Northern Ireland Join Ireland? I'd say there is a higher chance of Ireland being taken over by the UK again than that. Saying Ireland and Sinn Fein shot themselves in the foot during Brexit negotiations is a bit of an understatement. Firing a Ballistic Missile at their foot is a better description.

Ireland is in massive debt to the UK. UK has bailed Ireland out of Collapse too many times. Ireland can't produce enough food to feed it's people. Ireland imports most food from the UK. Ireland offers nothing of Value to the European Union. Apart from a Beach head for a Future invasion of the UK by the EU. If that even matters in this age.

See what I'm getting at? Ireland really has poked the bear. After the UK leaves the EU what is going to happen? Ask for repayments on it's loans. And stop exporting food to Ireland.

With how Ireland has treated the UK? The UK might repay them the favour. Let them collapse. Ireland is screwed when the UK gets No deal. Ireland might have no choice but to submit to the UK.