The Taoiseach has said a vote on a United Ireland in the wake of a no-deal Brexit would not be the right way forward.
Speaking at an event in west Belfast on Tuesday, Leo Varadkar warned it could divide people along sectarian lines and may be defeated or only narrowly passed.
He said: "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."
Mr Varadkar said equal standing for the competing political goals was at the heart of the Good Friday Agreement's concept of parity of esteem.
The Taoiseach recently faced criticism from the DUP for talking about the potential weakening of the Union in the event of a no-deal Brexit.
DUP leader Arlene Foster accused him of engaging in "Project Fear mark two" and deputy leader Nigel Dodds claimed he was breaching the 1998 peace accord by "talking up" the prospect of unification.
On a visit to Northern Ireland on Tuesday, Mr Varadkar said he hoped Brexit would not bring "constitutional questions to the fore".
Mr Varadkar said having a withdrawal agreement without the insurance policy mechanism to keep the frontier open if future talks foundered would not provide certainty.
Mr Varadkar has been a staunch supporter of the proposed backstop, which was agreed between the EU and former UK prime minister Theresa May's government.
Parliament's opposition to the measure saw Mrs May fail to get her proposal into law earlier this year.
Traders in Northern Ireland want some mechanism to keep the invisible land border open following Brexit and preserve the current friction less arrangements.
The backstop would mean the UK and Northern Ireland following EU rules relating to matters such as customs and trade.
Unionists believe it could threaten the integrity of the UK if Northern Ireland were to diverge from Great Britain.
Brexiteers feel it would keep the UK chained to the bloc indefinitely and could prevent it from striking trade deals with other parts of the world.
Mr Varadkar said he expected Prime Minister Boris Johnson would take up his invitation to visit Dublin in the next couple of weeks.