Theresa May and Jean-Claude Juncker agreed the outline of a divorce deal in December
Theresa May and the EU effectively fudged the Irish border issue in the Brexit divorce deal before Christmas.
But the commitments to leave the EU customs union, keep a soft border, and avoid divisions within the UK were always going to need reconciling at some stage. Currently 110million journeys take place across the border every year.
Both sides in the negotiations insist they want to avoid a hard border between Northern Ireland and the Republic, but their ideas for how the issues should be solved are very different.
If they fail to strike a deal it could mean a hard border on the island - which could potentially put the Good Friday Agreement at risk.
The agreement - struck in 1998 after years of tense negotiations and a series of failed ceasefires - brought to an end decades of the Troubles.
More than 3,500 people died in the 'low level war' that saw British Army checkpoints manning the border between Northern Ireland and Ireland.
Both London and Dublin fear reinstalling a hard border - whether by checkpoints or other means - would raise tensions and provoke a renewal of extremism or even violence if people and goods were not able to freely cross.
The DUP - which opposed the Good Friday Agreement - is determined to maintain Northern Ireland inside the UK at all costs, while also insisting it wants an open border.
The UK blueprint:
The PM has made clear her favoured outcome for Brexit is a deep free trade deal with the EU.
This would mean being aligned closely enough with the bloc that there is no need for customs