The European Union's chief Brexit negotiator Michel Barnier has said the EU will "stand fully behind Ireland" regardless of what happens in the Brexit negotiations.
He was speaking at a joint press conference with Taoiseach Leo Varadkar following a meeting at Government Buildings.
Mr Barnier said: "One thing is certain, whatever happens the EU will stand fully behind Ireland ... you have our full support".
He said that the backstop is the "only solution" to "maintain the status quo on the island of Ireland".
Mr Barnier also warned Britain that in case of a no-deal Brexit, the EU would not open trade talks until the question of the Irish border had been resolved.
"If the UK were to leave without a deal, let me be very clear, we would not discuss anything with the UK until there is an agreement for Ireland, Northern Ireland as well as for citizen's rights and the financial settlement," he said.
The Taoiseach said that Ireland is open to extending the Brexit deadline to allow talks in the UK to reach their conclusion.
He said he sincerely hoped it would be possible to find consensus in Westminster.
"A no-deal exit would be damaging for everyone." he said.
The meeting at Government Buildings took place ahead of a summit of EU leaders in Brussels on Wednesday.
The summit will decide whether or not the UK should be granted an extension to the Brexit deadline, which currently is due to expire on Friday.
Mr Barnier met Tánaiste Simon Coveney at the Department of Foreign Affairs earlier this afternoon.
The Taoiseach has made it clear that he is open to granting a second extension to the UK, as long as British Prime Minister Theresa May has a clear plan that is focused on a future relationship rather than trying to re-open the Withdrawal Agreement.
"What we want to avoid is an extension that just allows for more indecision and more uncertainty," he said.
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Sinn Féin leader Mary Lou McDonald said Brexit uncertainty is causing "real harm" on the island of Ireland and a customs union would not fully solve the Northern Irish border issue.
Speaking ahead of talks with Labour leader Jeremy Corbyn, she said: "There is a lot of talk now about an extension... and there's ongoing uncertainty which I'm sure has caused real problems in Britain and certainly has caused real harm on the island of Ireland."
Praising Mr Corbyn as someone who "understands Irish politics very deeply" unlike "in government circles", Ms McDonald said a customs union would only solve the issues around the Northern Irish border "to an extent".
She said: "It (a customs union) doesn't answer in its entirety the issue for the all-Ireland economy.
"Yes it is about the customs union but it's also about alignment to the single market."
Ms McDonald and the party's vice-president Michelle O'Neill also had discussions with Northern Ireland Secretary Karen Bradley.
They will also meet a number of Scottish National Party MPs at Westminster.
Additional reporting Micheál Lehane, PA