Since 8 November 2017, the Brexit negotiations have reached a stumbling block. The bone of contention: the Irish border.
A solution was found to the Irish issue in 1998. This involved, notably, free movement between the Irish Republic and Northern Ireland (a region in the United Kingdom). At the time, both countries (the United Kingdom and the Republic of Ireland) belonged to the European Union.
Those negotiating Brexit on behalf of the United Kingdom are envisaging re-establishing the border between the two countries which would annul the 1998 agreements and would inevitably relaunch the war.
Those negotiating Brexit on behalf of the European Union have proposed moving the economic border of the Union so that it does not, once again, divide the two Irelands but separates the island of Ireland from Great Britain. This would be tantamount to placing the economy of Northern Ireland under EU administration, a non-starter for the Crown.