The European Commission president said "Ireland will come first"
The European Commission President Jean-Claude Juncker has said "every line, every letter" of the Good Friday Agreement must be respected in Brexit negotiations.
He was addressing a joint sitting of the Dáil and Seanad in the Dáil chamber on Thursday.
Mr Juncker opened his speech by looking back at some "emotional, memorable moments" he has spent in Ireland - such as the 1996 Dublin Summit which paved the way for the single Euro currency.
On Ireland, Mr Juncker said: "Ever since Ireland took its rightful place in our union some 45 years ago, you have acted like a founding member state - often more so than some founding member states themselves".
"You have always sought the European approach: understanding that what is good for all in our union is good for us all individually.
"And Ireland itself has come a long way in that time - it went from a small, mainly agricultural economy to a thriving Celtic tiger in the '90s and 2000s.
"And thanks to difficult, tough decisions following the crisis, the economy's now more than back on its feet.
"The crisis took its toll on citizens and businesses alike, but together you have managed to turn the country around".
He also remarked that Ireland is the most youthful, globalised and optimistic country in the EU.
"The recent referendums on marriage equality and abortion reflect a deeper shift in societal views - a shift that would not have been contemplated even a generation ago".
"But perhaps the biggest change of all is that today's children can grow up in a peaceful land", he said, referencing cooperation between Ireland, the UK and the EU in the peace process.
On the British decision to leave the EU, Mr Juncker said: "Other member states, and Ireland in particular, should not pay the price for that choice.
"This is why when it comes to Brexit, I've always said that it is a case of Ireland first."
Acknowledging some progress in the talks, Mr Juncker commented: "The hardest parts are still to do, and there is not much time left to find a concrete agreement".
"On Ireland, both sides agree to the main principles: there should be no return of a hard border, we need common rules to preserve North-South cooperation.
"And mostly important, this means the Good Friday Agreement should be preserved in its entirety: every line, every letter".
"Whether you live in Derry or in Dundalk, the border has been out of sign and has been out of mind for 20 years - and that is how it must stay".
He said UK proposals for a temporary customs arrangement "show a certain willingness to make progress", but do not show how regulatory alignment would actually work.
"We have less than 10 months to Brexit - we need more answers, and fewer new questions", he said.
"I also want to be clear: Ireland will come first.
"There are those who think that the other 26 countries will abandon Ireland at the last minute for a special deal that suits them.
"Those people have not understood what being part of our union means: Ireland's border is Europe's border, and it is our union's priority", he said to applause from the chamber.
But he cautioned: "As the clock to Brexit ticks down, we must prepare for every eventually - including no deal.
"This is neither a desired nor a likely outcome, but it is not an impossible one - and we are getting ready just in case".
Responding to Mr Juncker's speech, the Taoiseach Leo Varadkar warned that the greatest challenge faced by Irish people on both sides of the border is to "unwrap the enigma, and solve the riddle of Brexit."
"We want to ensure that the future relationship between the EU and the UK is as close, comprehensive and ambitious as possible," he said.
"That is in our interests, and the interests of the EU as a whole - so we want to move into the detailed negotiations about that relationship as soon as possible.
"However, the backstop for the Irish border, agreed in December, must be legally operative in the withdrawal agreement, to apply 'unless and until' an alternative solution is agreed."
He noted that the "vision that delivered peace in Europe opened the door to peace in Ireland."
"The Good Friday Agreement was made possible by shared membership of the European Union and the single market," he said.
"It removed borders and differences between us.
"The removal of that foundation leaves us no less determined to protect the Good Friday Agreement, in all its parts, and in all that flows from it."
Mr Juncker's speech followed meetings with Mr Varadkar, Tánaiste Simon Coveney and Minister for European Affairs Helen McEntee on Thursday.
Mr Juncker held the talks at Government Buildings, discussing the state of play in Brexit negotiations in advance of a European Council meeting next week.
Other topics included trade, the Economic and Monetary Union, EU migration and the EU budget.
President Juncker was accompanied by the European chief Brexit negotiator, Michel Barnier.
This was his first visit to Ireland as president of the European Commission.
Additional reporting: Michael Staines