Top EU officials were warned Wednesday to make plans to cope with the fallout from a "no-deal" Brexit as their chief negotiator said talks with Britain must make more progress before next week's crunch summit.
There have been faint signs of progress amid reports that the two sides are now working on a single text of a divorce agreement - rather than making counter-proposals.
The UK Government is also due to publish its own revised proposals for the Irish border, but Downing Street has said only that these will appear "in due course".
DUP MEP Diane Dodds, who also took part in the meeting, said any type of trade barrier between Northern Ireland and the United Kingdom would be "disastrous".
Barnier said Brexit would trigger the need for customs, Value-Added Tax and compliance checks with EU standards between Ireland and Northern Ireland in the event that a planned "backstop" were triggered because a future EU-UK trade deal was not sufficient in itself to ensure the land frontier was not a "hard border".
The DUP is prepared to vote against Theresa May's Conservative Government on the budget should their red line on Brexit be breached, Sky News has reported.
How to prevent the return of a hard border, which some say could reignite sectarian divisions on the island of Ireland, has become one of the hardest hurdles to clear for both sides.
"The whole point about checks is that there is a difference" she said.
The DUP has dismissed the warnings about the peace process. He said customs declarations could be done online in advance.
But there are also likely to be increased regulatory checks in the Irish sea to protect the European Union single market.
Sources in the party - whose 10 MPs are propping up the PM in power - today upped the ante by threatening to vote down the Budget if the deal sees Northern Ireland and mainland Britain "treated differently".
Selmayr, secretary general of the European Commission, has been tasked with coordinating preparedness to protect the EU from economic and political damage if Britain leaves the bloc on March 29 next year without a divorce deal in place.
The European Commission also looked at preparations among the 27 states for multiple disruptions that this would involve, including customs checks and duties kicking in immediately on all imports and exports.
'There is a growing sense among the EU27 that outstanding preparedness questions have to be addressed very quickly and in a detailed way.
Speaking to business leaders in Brussels, Mr Barnier said negotiators were "intensively" working to find a deal. The sides will continue negotiations through the weekend with hopes for a breakthrough as early as Monday.