No deal will be as good as membership of the European Union, Jean-Claude Juncker has told the Irish Parliament.
The European Commission President addressed the Oireachtas on Thursday afternoon, and received applause when he stated the Irish border is also Europe's border.
Mr Juncker said Ireland "will come first" as Europe counts down to the UK's departure from the EU.
"There are those that think the other 26 countries will abandon Ireland for a deal that suits them," he said.
"Ireland's border is Europe's border and it is our priority."
"Of course it is in everyone's interests for the EU to stay as close as possible."
Mr Juncker reiterated his support for the nation, stating again that "Ireland is not alone".
"I am strongly against any temptation to try and isolate Ireland and not to conclude a deal on Ireland. Ireland has to be part of the deal," he said.
But he said that no deal is as good as membership of the EU.
"It simply does not exist," he said, adding that the prospect of the UK leaving the EU without any deal at all must be prepared for.
Responding to Mr Juncker's remarks, Taoiseach Leo Varadkar said he has secured a place in history amid the Brexit "riddle".
"This generation faces many challenges, not least the challenge of Brexit, which is starting to resemble a proverbial 'riddle, wrapped in an enigma'," Mr Varadkar said.
"History will record the important role President Juncker has played at a very difficult time.
"Your proven commitment to the European ideal will always find a home here, and our friendship will be steadfast as we work together towards a better Europe."
At the start of Mr Juncker's address, he joked that he wished he was drunk.
After finding some difficulty navigating steps, he assured members of the Irish Parliament: "I am not drunk, I have sciatica, I would prefer to be drunk."