Update - 1.02pm: Speaking after the EU gave the green light to the agreement Leo Varadkar said there was no plan B if the deal is not approved in the British House of Commons.
The Taoiseach said: "There isn't a plan B. The truth is what we have here is the best deal that is available both for the United Kingdom and for the European Union. We have a backstop in place but we all agree that that's an insurance policy that has to be there, but would only need to be invoked under certain circumstances."
He added that the UK would be "more than a third country" post-Brexit given its close links to the EU.
"We are determined to make sure that the United Kingdom isn't just another third country that we have a special relationship with the EU and UK afterwards," he said.
At a briefing with Irish media following the special meeting Mr Varadkar said that people should not be focusing on alternative options.
"Let's not forget what we have here, we have an agreement that took between 18 months and two years to negotiate, it is in several parts, is over 500 pages long and as of today has the support of 28 governments," he said.
"Anyone can have a better deal or an alternative deal in their own minds but an agreement 500 pages long that 28 member states can sign up to nobody has that.
Mr Varadkar said the "entire European Union" was of the view that the deal could not be renegotiated.
"It wasn't easy to get to this point," he said.
Mr Varadkar continued: "What's being put in front of EU parliament and House of Commons is a deal. Any other deal really only exists in people's imagination."
Asked whether EU leaders had discussed what they would do if the vote in the House of Commons failed, Mr Varadkar said the EU 27 had made a "conscious decision" not to discuss what may happen in such a scenario.
Mr Varadkar said the best outcome for all concerned would be if the UK remained in the EU, but given that was not going to happen the draft agreement was the "next best" thing.
"I've always thought the best outcome for Ireland and for Europe and indeed the United Kingdom would be for the UK to stay in the European Union, the customs union and the single market, but we respect the decision of the British people and the UK parliament not to do that," he said.
"What we need to do now is to get on with it and get on with the ratification of this agreement by the House of Commons and also by the European Parliament and when that's done, from March, April onwards, to get busy negotiating the future relationship treaty."
Theresa May has said British MPs will have to decide whether to back the draft Brexit deal and "move forward together into a brighter future" or reject it and "open the door to yet more division and uncertainty".
It comes after European Council president Donald Tusk confirmed that the 27 European Union leaders have endorsed the Withdrawal Agreement and the Political Declaration finally agreed with the commission last week.
European Commission president Jean-Claude Juncker has warned Britain cannot expect to get a better Brexit deal if the British Parliament rejects the agreement hammered out by Theresa May.
At a press conference in Brussels, British Prime Minister Theresa May said: "Before Christmas, MPs will vote on this deal.
"It will be one of the most significant votes that Parliament has held for many years. On it will depend whether we move forward together into a brighter future or open the door to yet more division and uncertainty.
"The British people don't want to spend any more time arguing about Brexit.
"They want a good deal done that fulfils the vote and allows us to come together again as a country.
"I will take this deal back to the House of Commons, confident we have achieved the best deal available and full of optimism about the future of our country.
"In Parliament and beyond it, I will make the case for this deal with all my heart and I look forward to that campaign."
Theresa May said she was not "sad" about Brexit as the UK and EU would remain "friends and neighbours".
Asked whether she shared Angela Merkel's sadness at Brexit, Mrs May said: "No, but I recognise that others do.
"I recognise some European leaders are sad at this moment, but also some people back at home in the UK will be sad at this moment."
But Mrs May said she was "full of optimism about the future of this country".
She added: "We will continue to have that good, close partnership and relationship within Europe, within the European Union and more widely."
Mr Juncker added: "I am totally convinced that this is the only deal possible. Those who think by rejecting the deal that they would have a better deal will be disappointed in the first seconds after the rejection of this deal."
The European Commission president said "divorce is a tragic moment".
"Payments have to be made but the future understanding is one which has to be constructed.
"I don't think Britain will be a third country like other third countries are third countries.
"There is, between us, something which has the remainings of love."