The Taoiseach Leo Varadkar has said any other Brexit plan "only exists in people's imaginations".
He was speaking after the withdrawal agreement was endorsed by the 27 EU leaders at a special meeting of the European Council in Brussels.
Leaders approved the deal just after 9.30am on Sunday.
President of the European Council Donald Tusk also said they supported the political declaration on the future EU-UK relations.
EU27 has endorsed the Withdrawal Agreement and Political Declaration on the future EU-UK relations.
— Donald Tusk (@eucopresident) November 25, 2018
Leaders first met with the President of the European Parliament, Antonio Tajani, and then attended a working session of the European Council.
Following on from this, they met for roundtable talks with the British Prime Minister Theresa May.
Speaking after the summit, Mr Varadkar insisted there is no alternative plan.
"Well there isn't a plan B.
"The truth is what we have here is the best deal that's available, both for the United Kingdom and for the European Union.
"We have 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 in certain circumstances.
"And even if it is, may only need to be invoked in part and for a temporary period as a bridge to a future relationship.
"And we all agree that the United Kingdom, after it leaves, will be more than a third country given the fact that it has had such a close relationship with the European Union that it is a member and will be the only former member".
“There is no plan B” says Leo Varadkar about the deal agreed today #Brexit pic.twitter.com/wNriyNdfVV
— Sean Defoe (@SeanDefoe) November 25, 2018
"In terms of alternative options and plan B and all of that type of thing, let's not forget what we have here: we have an agreement that took between 18 months and two years to negotiate.
"It, in several parts, is over 500 pages long and, as of today, has the support of 28 governments".
"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 - so what's on the table is the only deal that's on the table".
He added the "core difficultly" with people who argue there is a better deal on offer is "they don't agree among themselves what that better deal could be, they probably wouldn't have a majority in parliament for an alternative deal, either - and they certainly wouldn't have 28 member states signed up to it.
"Really what's being put in front of the European Parliament and the House of Commons is a deal - any other deal really only exists in people's imaginations".
"What we need to do now is to get on with it - 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, probably two treaties: one dealing with the economic partnership that will exist between the UK and the EU after the transition period ends, and the second one to deal with security".
President of the European Council Donald Tusk | Image: © European Union
Also speaking after the special meeting, Mr Tusk said: "The European Council approved the political declaration setting out the framework for the future relationship between the European Union and the United Kingdom of Great Britain and Northern Ireland.
"The European Council restated the union's determination to have as close as possible a partnership with the United Kingdom in the future.
"I quote this passage of today's conclusions, as it contains the essence of our meeting: Ahead of us is the difficult process of ratification, as well as further negotiations.
"But regardless of how it will all ends, one thing is certain - we will remain friends until the end of days, and one day longer".
President of the European Commission Jean-Claude Juncker speaks following a special meeting of the European Council in Brussels | Image: © European Union
While European Commission President Jean-Claude Juncker also said this deal is "the only deal possible".
"This is the best deal possible for Britain, this is the best deal possible for Europe, this is the only deal possible - the only deal possible".
"I'm convinced that those having to ratify this deal and in Westminster and in the European Parliament will have this in their minds when they are taking their individual decision".
He also said that no civil servants in the Commission who are from Britain "will be pushed out".
UK PM Theresa May sits next to European Council President Donald Tusk during roundtable talks with the EU 27 leaders in Brussels | Image: © European Union
A general view of roundtable talks with the EU 27 leaders and UK PM Theresa May in Brussels | Image: © European Union
Earlier, Mr Varadkar insisted the deal protects Irish Interests, citizens and the economy.
"We spent two years trying to negotiate an agreement which protects our interests - our citizens and our economy - and I believe we have that.
"An agreement which allows for an orderly withdrawal of the UK from the European Union, an agreement which protects the freedoms and rights of citizens - particularly the Common Travel Area - an agreement which provides a transition period during which we can negotiate a future relationship.
"And if that isn't successful, then a backstop that kicks in that assures us that there'll be no hard border between Northern Ireland and Ireland and we'll continue to have tariff free and quota free trade between Britain and Ireland - which is very important for our economy."
The deal sets out how the UK will leave the EU over the next few years - and what happens if both sides fail to reach a new trade agreement in that time.
The agreement by EU leaders paves the way for votes in the European Parliament and the British House of Commons.
Although Mrs May is facing a massive challenge to get the deal passed in her parliament - and if she cannot, it is back to square one for negotiations.
She faces challenges from the DUP, UK Labour and rebels within her own Conservative Party.
Michel Barnier arrives for a special meeting of the European Council in Brussels | Image: © European Union
The EU's chief Brexit negotiator Michel Barnier has said the bloc will remain an ally of the UK.
"We have worked, thanks to the full unity of the 27 head of states, the member states, their personal trust and also as well with the European Parliament.
"I have worked with my team and (have negotiated) with the UK, never against the UK. And I want also to thank the British team.
"Now it's time for everybody to take their responsibility - everybody.
"This deal is a necessary step to build the trust between the UK and EU - we need to build in the next phase this unprecedented and ambitious partnership.
"We will remain allies, partners and friends".
Additional reporting: Sean Defoe
