Because (depending on the exact poll) either a majority of plurality of the people of Northern Ireland favor remaining in the UK, so it’s not that simple politically. The Good Friday Agreement has provisions for a change in the future status of Northern Ireland based on a referendum, so if/when a majority of its population want to join the Republic of Ireland, this can happen.

Northern Ireland receives heavy subsidies from the UK government. I imagine the UK would be quite happy to cede it to the Republic of Ireland were this politically feasible and not likely to kick off another round of sectarian violence. Ireland may want the six counties in principle, but might be a little unhappy with the bill.

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