People, money, Ireland. These are the three big questions on which the immediate future of the Brexit project hinges. When European Union leaders meet in October, they will decide whether “sufficient progress” has been made in talks with the British to allow for the opening of substantive negotiations to determine the United Kingdom’s relationship with the EU after it leaves in March 2019. As the EU’s lead negotiator Michel Barnier put it last May:

I…made very clear that the [Irish] border issue will be one of my three priorities for the first phase of the negotiation. Together with citizens’ rights and the financial settlement. We first must make sufficient progress on these points, before we start discussing the future of our relationship with the UK.

By the border issue he means the question of whether a hard customs and immigration border is to be imposed between Northern Ireland and the Irish Republic.

And so the Irish Question rises yet again, looming on the road to Brexit like the Sphinx on the road to Thebes. It threatens to devour those who cannot solve its great riddle: How do you impose an EU frontier across a small island without utterly unsettling the complex compromises that have ended a thirty-year conflict? The “people” part of the preliminary Brexit negotiations concerns the mutual recognition of the rights of EU citizens living in the UK and vice versa. The “money” part concerns Britain’s outstanding obligations to the EU budget and the calculation of the final divorce bill. Both are awkward and politically divisive issues, but it should be perfectly possible to reach a settlement.

http://www.nybooks.com/articles/2017/09/28/brexits-irish-question/