Taoiseach Leo Varadkar has said he regrets that the UK is leaving the European Union but said that what was agreed today is the "next best outcome".
He was speaking at a press conference after all 27 European Union leaders endorsed the Brexit deal at a summit in Brussels.
There is no plan B, he added, if the deal is not approved in the House of Commons.
He 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."
The Taoiseach 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.
The Taoiseach 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.
He added: "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. What's on the table is the only deal that's on the table."
The UK would be "more than a third country" post-Brexit given its close links to the EU, he said.
"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.
Mr Varadkar said that the idea that a better Brexit deal can be negotiated between Britain and the EU is "imaginary".
Speaking on RTÉ’s This Week programme, he said there is still time to convince British MPs to vote in favour of the agreement approved in Brussels this morning.
The Taoiseach rejected suggestions that there is no prospect of getting the Withdrawal Agreement through the British parliament and warned that there is "no alternative" that would command the support of member states.
"As every day goes by, more and more voices of business, of farmers, of civil society in Britain and Northern Ireland are speaking up in favour of this deal," he said.
"The vote hasn't happened yet and there is time over the next few weeks for minds to focus."
He said: "The real choice that is facing MPs in a few weeks' time is a stark one - It is between this deal which is on the table, on the one hand, or no deal at all."
Asked if he was concerned that the backstop could be up for negotiation again if British Parliament rejects the deal, Mr Varadkar said: "What has been agreed is a package. So if you start to pull at one string then the whole thing unravels.
"Anyone can imagine in their own minds the deal that would be the perfect deal for them, but can they actually produce an alternative deal one that would have the support of 28 governments? I don't think so.
"So this idea that there is a better deal that can be negotiated is largely imaginary and we've had enough imagination and fantasy thinking when it comes to Brexit already."
Additional reporting: Mary Regan