By Aine McMahon, PA
Sinn Fein president Mary Lou McDonald has said UK Prime Minister Boris Johnson is following a Brexit "fantasy", and that his plan is "profoundly stupid and immeasurably dangerous".
Speaking at her party's away day in Dundalk, Co Louth, Ms McDonald said: "British Prime Minister Boris Johnson follows a fantasy in which the rights and interests of the Irish people can be cast aside.
"It is time to call Mr Johnson's fantasy for what it is - profoundly stupid and immeasurably dangerous."
She added: "Mr Johnson's stupid, dangerous fantasy cannot become Ireland's nightmare, because Brexit is a very English problem and the consequences of it cannot be shifted on to Ireland."
Ms McDonald also said Ireland faces "its greatest threat and challenge in a generation".
"As October 31 approaches, neither an Taoiseach (Leo Varadkar) or (EU chief Brexit negotiator) Michel Barnier can blink in the face of Tory intransigence, because too much is at stake," she said.
Ms McDonald said Mr Johnson's Brexit plan would lead to a hard border on the island of Ireland.
She said: "Despite Mr Johnson's claims to the contrary, the path he is leading would lead us to a hard border. The British Prime Minister needs to hear this.
"The Irish people will not allow our country, our communities, our economy or our peace to be vandalised by him.
"There can be no return to a hard border, customs checks, and no more division on this island to suit the needs of a British Government that has no interest in Ireland's future."
DUP leader Arlene Foster has said her party was working with Mr Johnson to find a deal that sees Northern Ireland leave the EU with the rest of the UK.
Ms Foster said a Northern Ireland-only backstop would mean one part of the UK would be left in the customs union and subject to the rules of the single market without any consent.
But Ms McDonald said "no viable alternative exists to the backstop".
"It is not perfect by any means but it's the least worst option and broad Irish opinion supports that," she said.
"This isn't about us stamping our feet, this is a compromise solution, a minimum insurance policy.
"I believe that despite DUP rhetoric to the contrary, broad Irish opinion north and south understands that and supports that.
"If the DUP were representing northern citizens and society well they would recognise that. They are very much adrift from the consensus position in the north and the south."
Ms McDonald added that preparations must now be made for a united Ireland and that her party will run on an abstentionist ticket.
"No true Irish republican would ever take a seat in the British Parliament, much less swear an oath to the British crown," she said.