The former Prime Minister of Ireland has rejected the suggestion the country could follow the UK out of the European Union, saying: "We're mad, but we're not that mad."
Bertie Ahern said there was no appetite "to get involved in an act of insanity" to ensure freedom of movement following Brexit.
DUP MP Ian Paisley had claimed Dublin would be forced to choose between a hard border with Northern Ireland or quitting the EU, amid an impasse over the nature of the frontier.
Mr Ahern, Ireland's Taoiseach for 11 years until his resignation in 2008, admitted there was no "easy solution" to the problem.
But he told Sky News: "There's no prospect of the Republic of Ireland leaving the European Union. That's not going to happen. There is no support, thankfully, for us to get involved in an act of insanity."
He added: "We're mad, but we're not that mad. And we're not going to do that now or in the future - and never."
Mr Ahern has previously warned Brexit could pose a threat to Northern Ireland's peace process.
His latest comments came after Northern Irish MPs vowed to fight Dublin's demands for a sea border with Britain after Brexit.
Ireland's new Taoiseach, Leo Varadkar, has also reportedly warned Theresa May a land border could jeopardise the peace process and restrict freedom of movement.
British ministers had proposed using measures like surveillance cameras to allow movement between the north and south of the island.
But Dublin believes a land border would be unworkable, saying border checks should take place at ports and airports, effectively drawing a new frontier in the Irish Sea.
Reacting to reports of the demands, DUP MP Ian Paisley tweeted: "1 of 2 things will now happen 1. A very hard border 2. Ireland will wise up and leave the EU."
His party colleague Jeffrey Donaldson said requiring customs and immigration checks for travel between parts of the UK would be "absurd" and "unconstitutional".
"There is no way that the DUP would go for an option that creates a border between one part of the United Kingdom and the other," he told the BBC's Today programme.
Irish foreign minister Simon Coveney, speaking at a meeting of EU foreign affairs ministers last week, said: "What we do not want to pretend is that we can solve the problems of the border on the island of Ireland through technical solutions like cameras and pre-registration and so on. That is not going to work."
He added: "Any barrier or border on the island of Ireland in my view risks undermining a very hard-won peace process and all of the parties in Northern Ireland, whether they are unionist or nationalist, recognise we want to keep the free movement of people and goods and services and livelihoods."
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