The taoiseach (Irish PM) has said a vote on Irish unity in the wake of a no-deal Brexit would be "divisive" and "not the right way forward".
Leo Varadkar was speaking during a debate at west Belfast festival Féile An Phobail.
Earlier, he said he still believed a no-deal Brexit could be avoided.
But he said a border poll following a no-deal could result in some of the mistakes made during the partition of Ireland being repeated.
"I think it would result in some of the mistakes made 100 years ago, when partition happened, being repeated but just the other way around - a huge number of people, those from a unionist, British, Ulster background, being brought into a united Ireland against their will."
He added that without the necessary preparation, it would "break down on sectarian lines" and "there's a chance it would be defeated".
Mr Varadkar said if his government prepared for a border poll, it would be "counter productive" as he has made efforts to persuade unionists that the EU withdrawal agreement - including the backstop - has been about protecting the status quo and not an attempt to bring about constitutional change.
Analysis: Border poll calls divide opinion
By Mark Devenport, BBC News NI Political Editor
Although she suggested something to the contrary in an interview a year ago, in recent months Sinn Féin president Mary Lou McDonald has been consistent that a no-deal Brexit should trigger a border poll.
It's no surprise that the taoiseach disagrees. He argues that Northern Ireland cannot be simply grafted on to the Irish Republic in the way that East Germany was assimilated into West Germany.
Instead, Leo Varadkar says an entirely new state with a new constitution would be required, which takes into account the British identity of a sizeable section of its potential future population.
All this would require careful and lengthy preparation.
Ms McDonald turned this argument on its head at the leader's debate, insisting that if the Irish government isn't prepared for the prospect of a United Ireland, then it needs to get prepared.
Earlier on Tuesday, Mr Varadkar met business leaders to discuss Brexit as he visited Northern Ireland for the second time in four days following his participation in Belfast's Pride parade on Saturday.
At Hillsborough Castle he was greeted by DUP MP Sir Jeffrey Donaldson, as well as business and tourism officials.
Asked during a press conference if he accepted a no-deal Brexit was now likely since the new UK prime minister had taken office, Mr Varadkar replied that a no-deal outcome could still be avoided.
He said this could happen by ratifying the current withdrawal agreement, agreeing a further extension to the deadline, or revoking Article 50: the mechanism that triggered the Brexit process.
Boris Johnson has ruled out any of those options, however Number 10 has denied it is unwilling to negotiate with the EU and wants talks to fail.
Speaking later at Féile An Phobail, Mr Varadkar said it was "not true" that the EU was unwilling to talk to the UK government.
He added that while the EU said the withdrawal agreement was not open for renegotiation, it was prepared to discuss the political declaration with the UK.
The political declaration sets out the future relationship between Britain and the EU after Brexit.
The taoiseach also said on Tuesday that he understood some people had become "weary" of Brexit and feel the UK should leave the EU by 31 October "come what may".
But he added that, even if a no-deal outcome happens, negotiations would need to begin at some stage anyway to try to resolve the Irish border issue.
"Brexit is not a storm we weather or prepare for, it is a permanent change and that needs to be borne in mind," he said.
He also said it was "not true" that the EU was unwilling to talk to the UK government, contrary to what Michael Gove said.
DUP deputy leader Nigel Dodds said his party wanted to see a "sensible deal" between the UK and the EU, but that this could only happen if "Dublin and Brussels are in deal-making mode".
Stop 2 for Taoiseach Leo Varadkar in NI as he arrives at the Duncairn centre for discussion with young professionals and business, church and community leaders as part of Ireland Funds event - describes the Hall as a “nice space” @BBCNewsNI pic.twitter.com/s4dsM7eypY— Jayne McCormack (@BBCJayneMcC) August 6, 2019
Ireland's Finance Minister Paschal Donohoe also met Chancellor Sajid Javid in London for Brexit talks.
His visit came with continuing uncertainty over the status of UK-EU negotiations over Brexit.
Speaking on the BBC's Newsnight programme, Mr Donohoe said "the prospect of a no-deal Brexit has grown" since Boris Johnson became prime minister.
"I believe no deal is a very credible and material risk now and I believe Prime Minister Johnson feels differently about the relationship between the UK and EU and the future trust of that relationship [compared to] how Prime Minister May would have," he said.