The Taoiseach has declared his opposition to a border poll in the immediate aftermath of a no-deal Brexit.
Leo Varadkar told an audience in west Belfast that it would not be constructive and could be defeated, or could win by only a slim majority.
He was taking part in a leaders' debate at Feile an Phobail with Sinn Fein president Mary Lou McDonald, DUP MP Gregory Campbell, UUP MLA Doug Beattie, Alliance leader Naomi Long, SDLP MLA Daniel McCrossan and Fianna Fail parliamentary party chairman Brendan Smith.
During the debate, Mr Cambpell strongly defended the DUP's confidence-and-supply deal with the Tories.
He said there were "a billion reasons" to be proud of it and it had benefited everyone in Northern Ireland.
There was standing room only at the event in St Mary's College with two overflow rooms and people trying to listen to the debate in the grounds outside.
Mr Varakdar said that a border poll following a hard Brexit would not be "the right way forward".
It would take place at a very chaotic period when the Republic was still sorting out its future relationship with the UK.
There may well not be a functioning Assembly or Executive at Stormont, nor prior discussion about what Irish unity means. The Taoiseach said it was important that the six counties were not just absorbed into the 26 counties - as East Germany was into West Germany - "it would have to be a new state".
He said a border poll in such "chaotic" circumstances would be divisive, with voting following sectarian and religious lines.
There was also a danger it would be defeated, which would be "a real shame" for those who believe in Irish unity. Even if it won by 51-53%, it would be nowhere near the 71% endorsement of the Good Friday Agreement.
Mr Varadkar said he did not want what happened after partition repeated in reverse, with the unionists being alienated from the state.
Ms McDonald strongly supported a border poll. "Brexit has demonstrated that partition is a disaster," she said. The constitutional question was on the table whether "people choose to bury their heads in the sand or not".
Mrs Long said the likelihood of a border poll was increasing with the chances of a hard Brexit. But she didn't support one in those circumstances.
"What we need is order, what we need is certainty," she added.
When asked if the DUP trusted Prime Minister Boris Johnson on the backstop, Mr Campbell said: "It's not a matter of trust. I don't have to trust Boris and Boris doesn't have to trust us.
"What we do have to do is business with each other, and that business has brought benefit to everyone."
He said those who "sit and lambast" the confidence-and-supply deal hadn't delivered anything for anyone in Northern Ireland.
He also said whether the Taoiseach or Sinn Fein president liked it or not, his British identity wasn't up for negotiation.
Mr McCrossan said the Prime Minister was "full of bluff and bluster" and he expressed concern about his "reckless approach". He branded the Cabinet "extreme Brexit lunatics".
Ms McDonald said the British were "very wrongly pointing the finger" at the Taoiseach for the current Brexit morass when the blame lay with the Tories.
Mr Beattie said that, as a Remainer, he accepted the referendum result. He could not entertain a backstop "in any shape or form" which would override the Union.
He argued that it should be scrapped or amended. "Is the backstop worth a no-deal Brexit?" he asked. "Boris is bullish, trust me. I don't trust him but he is bullish. We have to come up with common ground and meet in the middle."
Mr Beattie said it was too easy to look for a scapegoat and blame Mr Johnson, Nigel Farage, the Taoiseach or Michel Barnier. He called for a return to the spirit of the Good Friday Agreement to resolve the paralysis.