"Backstop is needed in order to respect the integrity of the EU single market and the EU customs union", he said, adding that goods entering Northern Ireland must comply with the rules of the single market and the customs union code.
"Only Northern Ireland-specific solutions will work", he added.
Barnier hit back at claims by the leader of the Democratic Unionist Party (DUP) Arlene Foster that he was not an "honest broker", saying he was the "negotiator for the 27".
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Mr Barnier was speaking at the start of a two-day visit to Ireland. The border issue has been cited as the chief hurdle to be overcome in the Brexit talks.
"Michel Barnier's trying to present himself as someone who cares deeply about Northern Ireland and if that is the case he needs to hear the fact that we are part of the United Kingdom; will remain part of the United Kingdom constitutionally, politically and economically", she told the BBC.
With the Prime Minister set to decide her customs policy at a crunch meeting on Wednesday, Mr Barnier tried to pile on pressure demanding an agreement on the Irish land border question by June.
But former Stormont finance minister Mr Wilson said Mr Varadkar had behaved in "an appalling way towards Northern Ireland" and had encouraged the European Union to insist on requirements in the December Brexit agreement which would have "in effect have separated Northern Ireland from the rest of the UK".
"Without backstop, there can be no withdrawal agreement", he said.
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Last year, the first jury to consider the matter wasn't able to reach a verdict, and Judge Steven O'Neill announced a mistrial. As Constand stood silently behind him, Steele apologized to her for a previous DA's decision in 2005 not to charge Cosby .
He inadvertently had a dig at Davis when he took a question from a Northern Irish journalist about Davis believing the backstop wasn't necessary. The backstop was seen as hugely controversial because it would keep Northern Ireland in the customs union and Single Market - something which Theresa May said would never be acceptable.
"We want to succeed with the United Kingdom, not against the UK".
"The Irish government however is less keen to engage with the families of those murdered by republican terrorists in the same areas - particularly where there is suspicion of state collusion".
Mr Barnier visited the Irish border town of Dundalk, Co Louth, for a conference organised by the Irish Government on Monday.
Both Mr Varadkar and Irish Foreign Minister Simon Coveney are still doing this, the DUP MP said, "and then today he waltzes into Northern Ireland as if he owned the place".
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