"With the depreciation of the sterling, Brexit is already having an impact on the value of Irish exports to the United Kingdom, in particular the agri-food sector, and many in Ireland fear the return of tensions in the North".
"The very same fact that it has taken them more than nine months to start negotiations after the results of the referendum tells you it is much more complicated than they calculated".
He said: "This island has strong allies in Europe".
"But as I already said many times, nothing in this negotiation should put peace at risk, nothing". "We have together the duty to speak the truth", Barnier said. "There are really important and hard challenges in this. We have to deal with it in an orderly and constructive manner", he said.
Speaking earlier today at the European People's Party at Druid's Glen in Co. The media was not permitted to attend the conference session with Mr Blair.
But following the EPP meeting later he stressed that he believed there was a "common desire" to treat Northern Ireland as a "special case".
"The Common Travel Area has meant ease of going back and forth across the border, vital for work and family connection has been in place for nearly 100 years.
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The UK's departure from the European Union will have consequences".
Irish Prime Minister Enda Kenny has said any semblance of a return to physical infrastructure along the border that was marked by military checkpoints until 1998 could lead to a renewal of the armed conflict.
Barnier, upon whom Dublin has focused much of its diplomatic offensive, will see the issues first hand on Friday when he travels with Ireland's Foreign Minister to visit some food businesses and farmers who work along the border. "That open border is a must in all circumstances", he said.
Michel Barnier has said that an answer to the issue of the Irish border after Brexit can be found if the political will exists.
"The customs union never came into the debate until January 17 when Prime Minister May raised the customs union issue. And the absence of customs controls - both countries being in the Single Market and Customs Union - have meant a huge boost to UK-Irish trade".
Manfred Weber, chair of the EPP group, said Brexit can not be a "win-win" for the UK.
Brexit Secretary David Davis has said the United Kingdom would adopt technology to cover the transport of goods between Northern Ireland and the Republic.