Leo Varadkar has said that more and more people in Northern Ireland "will come to question the union" in the event of a no-deal Brexit.
The Taoiseach also signalled that the Irish government would have to consider a white paper or a forum on Irish unity in the event of the UK leaving the EU without an agreement. However, he cautioned that to do so now would be "provocative".
Mr Varadkar told the MacGill Summer School in Glenties, Co Donegal, that a hard Brexit "raises very serious questions about the future of Northern Ireland".
He said he didn't know if he would see a united Ireland in his lifetime, but that liberal Protestants and unionists would start to question whether they feel at home in Britain that is talking about the return of the death penalty - a reference to previous remarks made by new Home Secretary Priti Patel.
He said: "I do think more and more people certainly in the event of no deal, more and more people in Northern Ireland will come to question the union.
"People who you might describe as moderate Catholics or moderate nationalists who are more or less happy with the status quo will look more towards a united Ireland and I think increasingly you see liberal Protestants and liberal unionists starting to ask the question as to where they feel more at home.
"Is it in a nationalist Britain that's talking about potentially bringing back the death penalty and things like that or is it part of a common European home and part of Ireland?
"I think one of the things ironically that could really undermine the union is a hard Brexit both for Northern Ireland and for Scotland and that's a problem they're going to have to face."
Mr Varadkar also said that while the DUP are "in a privileged position at the moment" in holding the balance of power in Westminster, they must be "smart enough to realise that the kind of influence they have now is probably not going to last very long".
The Taoiseach also said the UK will have brought the potentially dire consequences of a hard Brexit on themselves in the event of no-deal.
In a sign of the Irish government taking a more critical tone with new Prime Minister Boris Johnson, Mr Varadkar said the UK alone would be to blame for a no-deal situation
In Belfast, Tanaiste Simon Coveney said Mr Johnson's approach to Brexit thus far in demanding the complete removal of the backstop was "worrying for everybody".
He described his comments since taking office as "very unhelpful" and setting Britain on a "collision course" with the European Union and Ireland.
Speaking later in Co Donegal, Mr Varadkar said: "If the UK leaves on October 31 that is their choice. There may be an attempt in the United Kingdom, to somehow blame European Union, or somehow blame Irish inflexibility, that is totally wrong.
"The UK can stop no deal at any time, they can revoke Article 50, it can request an extension for a good reason, it can ratify the withdrawal agreement.
"So if no deal happens on the October 31, whatever consequences are for the British economy, for the union and for Ireland, they will be things they brought on themselves."
Mr Varadkar repeated that there would be no Brexit deal without the backstop and that removing it would be the same as a no-deal scenario.
He said that the Irish government should be "wise to the possibility" that the new Prime Minister Boris Johnson may call a snap general election in September, October or November.
His remarks came shortly before Mr Johnson ruled out a pre-October 31 poll.
In a further jab at the political turmoil in the UK, Mr Varadkar said the political stability in the Republic since the Irish general election in 2016 was crucial with Brexit looming.
"Most people, including myself, believed that it wouldn't last very long and we're now on our third British Prime Minister already. There could be a fourth yet, who knows," he added.
Mr Varadkar said that Mr Johnson had demonstrated flexibility and, to laughter from the audience, noted he had voted both for and against the withdrawal deal.
"The real thing I would like to do is just get a sense from him as to what he's thinking and what his plans are. He's demonstrated a degree, I think, of flexibility in the past having voted both for and against the Withdrawal Agreement. I don't think he's going to be entirely inflexible in the future," he said.
Mr Varadkar said he looked forward to "getting the measure of the man" and hearing what his plans are.