Bertie Ahern claims any kind of border between the Republic and the North would be a disaster.
The former Taoiseach has complimented current leader Leo Varadkar for standing firm in Brexit negotiations - but is worried British Prime Minister Theresa May does not understand the significance of the deal for Ireland.
Tomorrow marks a watershed moment in Anglo-Irish relations with the 20th anniversary of the Good Friday Agreement which helped bring peace to the North.
Bertie Ahern says nothing should jeopardise it.
He said: "Any kind of a physical border that brings us back to anything that we had from the 1920s on would be a disaster."
Bertie Ahern claims issues holding up power-sharing in the North aren't 'insurmountable'.
The former Taoiseach says politicians trying to return a Government to Stormont are dealing with much smaller issues than those faced by the authors of the Good Friday Agreement.
The peace deal will mark its 20th anniversary tomorrow.
Bertie Ahern says it was far harder than the current crisis.
He said: "The kind of issues that were on the table for us were the release of prisoners, the decommissioning of weapons, new legalisation on criminal injustice, huge issues.
"The kind of issues on the table now are not insurmountable."
This week marks the 20th anniversary of the signing of the Good Friday Agreement.
It comes at a time when Brexit places a question mark over the border - and power-sharing grinds to a halt.
The landmark 35-page deal brought about a promised end to the violence of the troubles but is now in the spotlight with the prospect of Brexit.
The very structures it created to allow for power-sharing at Stormont have now continued to lie idle for over a year.
Events have already taken place in Washington and New York to mark the historic peace treaty.
And tomorrow (the actual date of the agreement) former US President Bill Clinton and former US senator George Mitchell, who helped broker the deal are due to accept the Freedom of Belfast at an event in the city.
George Mitchell yesterday told the BBC that the single most important result of the Good Friday Agreement was 20 years of peace and he denied claims by some that it was now getting in the way of political progress in the north.
Speaking in advance of the anniversary Sinn Féin leader Mary Lou McDonald has restated Sinn Féin's commitment to the agreement and urged all the other parties, including the UK and Irish governments, to recommit to the Agreement, power sharing, reconciliation and progress.