A post-Brexit "hard" border between the Northern and the Republic of Ireland must not be allowed to happen, political leaders from both countries have warned.
People and goods going between the two nations have been able to move freely thanks to the Common Travel Area (CTA).
The open borders agreement, set up in the 1920s, has been strengthened by both Britain and Ireland's European membership.
However, questions and concerns have been raised about what this means for the CTA and for both economies in the wake of the UK's Leave referendum result.
The issue was one of the hot topics discussed at an emergency meeting of the British Irish Council in Cardiff - which was called by Wales' First Minister Carwyn Jones.
Among heads of government attending the talks were Ireland's Taoiseach Enda Kenny, Scotland First Minister Nicola Sturgeon as well as Northern Ireland political heavyweights Arlene Foster and Martin McGuinness.
Mr Kenny said it was vital there would not be a return to the days of "checkpoints, towers and customs".
He said: "There are modern ways of not having hard borders.
"I can tell you that hard borders would not be accepted in the south or the north.
"The best possible outcome for everyone is a UK that's prosperous and has the closest possible links with the European Union.
"For us in Ireland, that means access to the single market.
"And it's been made perfectly clear by the EU Council that if that is to happen, then the respect has to be there for the freedom of (movement of) people.
"We have difficulties but I expect us to retain the Common Travel Area.
"It's a fundamental part of who we are."
Leaders and ministers from eight BIC members - the UK and Irish Governments, devolved administrations of Northern Ireland, Scotland and Wales and the governments of Jersey, Guernsey and the Isle of Man - met in the Welsh Government's Cathays Park headquarters on Friday.
Welsh Labour leader Mr Jones said he called an extraordinary meeting of the council at short notice because of the "tumultuous" political landscape after the EU referendum.
He said: "The council plays a unique and important role in developing positive relationships between its members.
"It is more important than ever to maintain the strength of this relationship and work together to map out a successful way forward."
Also heralding the talks as the council's most vital to date was Northern Ireland Deputy First Minister and Sinn Féin MLA Mr McGuinness.
The 66-year-old said he had "no faith" in a Conservative Government in Westminster to replace any money lost by being out of the European Union.
He also said he remained concerned by Theresa May's comments last month about the possibility of a new border between Northern Ireland and the Republic.
In June, during a Remain vote campaign, the then home secretary said it would be "inconceivable" for there not to be border controls when going between the two countries.
Mr McGuiness added: "I have since spoken to (Northern Ireland Secretary) James Brokenshire, who appears to have a different position. But it will be important to find out the British Prime Minister's view, because if her position is still the same it represents a very serious issue."
Northern Ireland First Minister Mrs Foster argued that the establishment of checkpoints in the 1970s had been due to terrorist threats - but said she believed a Common Travel Area would still be possible even with the UK out of the EU.
She said: "It's not only vital to Northern Ireland, the Republic of Ireland, but also (other home nations such as) Wales too." this email
The council's debate over the Common Travel Area come after Francois Hollande's said on Thursday that he was prepared to recognise Ireland's "special" circumstances.
The French president had also urged Britain not to delay the process of negotiations to pull the country out of the bloc - a sentiment echoed by Taoiseach Mr Kenny.
Wales' First Minister Mr Jones said Article 50 must be triggered before next summer - or there would be a real a risk Leave voters would think the referendum result was being ignored.
However, the former barrister said there were still a lot of issues to resolve first.
He added: "There are some issues - like farming or fisheries - where in policy terms there is no UK. It's all entirely devolved.
"So any devolution in those fields, the UK Government can't negotiate or agree on behalf of the devolved governments.
"Yes, people did vote to Leave in Wales, and that's not going to change. But the reality is we still have issues we need to resolve.
"We are going to lose £600 million a year of European funding. A promise was made by some in the Leave campaign that that would be made up. Well, we've not got that money yet nor the promise of it.
"I want to make sure we continue talking to the European Commission and try to get the best deal for Wales, bearing in mind that the people of Wales voted to Leave.
"People voted to Leave. They didn't vote to be done over financially."
Ms Sturgeon described the talks as very "frank and very robust" and said she would ensure Scotland played a full part in the Brexit discussions.
She added: "We are in unprecedented times. And I think that we have to be prepared to think about unprecedented solutions to the circumstances we found ourselves in.
"If there's not a way of doing that within the UK, then it sends the message to people in Scotland that our voice doesn't matter and can be ignored.
"The UK is a multinational state and if it's not able to demonstrate that the voices of the different nations can be heard and listened to, then certain conclusions will be drawn from that.
"There's an onus (on the UK government) to demonstrate to the rest of the United Kingdom that solutions can be found.
"If that doesn't happen, then for Scotland other options will have to be considered."