Fianna Fáil leader Micheál Martin says political parties should not confuse Irish unity with Brexit.
He says the Good Friday Agreement is the roadmap for a United Ireland and he has always challenged Sinn Féin's call for a "border poll".
Deputy Martin said a unity poll could inflame tensions by forcing people into a rushed decision, adding that restoring Stormont is the way to bring people together
"Before you want to talk about new institutions or new approaches, you have got to demonstrate that you can work the existing ones and they haven't," he said.
"That is very disappointing and I think what is happening in the North is there is an emerging middle ground.
"I think that we should nurture that middle ground and see where that takes us."
Sinn Féin TD Pearse Doherty has criticised Mr Martin's comments calling them shameful.
Mr Doherty accused Fianna Fáil of abandoning nationalists in the North.
"I think that they have finally dropped any pretence of being a republican party and many could claim that they are acting anti-republican," said Deputy Doherty.
"At a time when there is growing demand for Irish unity and when there is growing demand for governments to start preparing for a referendum and the transition that is needed to bring that about."