Bertie Ahern has warned now is not the time for a border poll on Irish unity.
The former taoiseach, who played a key role in the Good Friday Agreement, said a "sectarian headcount" in Northern Ireland is the last thing that is needed amid the fall-out of Brexit.
"The last thing I want out of Brexit, the very last thing, is anyone (going) on about border polls," he said.
"The only time we should have a border poll in my view, and I will argue this for the rest of my life, is when we are in situation where nationalists and republicans and a sizeable amount of unionists and loyalists are in consent.
"To have a sectarian headcount now is the last thing we need."
But Mr Ahern said the provision for Irish reunification, enshrined in the Good Friday peace deal, should be stitched into any Brexit deal between the UK and the EU.
Speaking before a special Brexit committee of the Seanad, Dublin's upper house of parliament, he said those in favour of a united Ireland should continue to work towards a future time when a border poll would be appropriate.
He added: "This is not the time for it, there will be a time for it."
Sinn Fein, who made gains in the recent Stormont elections, has insisted there is an urgent need for a reunification referendum after the UK vote to pull out of the EU.
The majority of voters in Northern Ireland backed remaining within the EU - 56% to 44%.
But the Democratic Unionist Party, the region's largest, has accused Sinn Fein of creating further uncertainty and division in calling for a border poll.