Preparatory work which would allow a Border poll needs to take place "sooner rather than later", former taoiseach Bertie Ahern has said.
Mr Ahern told the Seán MacDiarmada summer school that the Belfast Agreement allowed for Irish unity by consent and that, despite 100 years of partition, no preparation had taken place for such a poll.
Mr Ahern said the “Brexit fiasco” demonstrated that an “it will be alright on the night” approach to a Border poll would not work.
"For the avoidance of any doubt, that we need to flesh out and have agreement by what is meant by a united Ireland is not a recipe for 'kicking the can down the road'," he told the school which remembers MacDiarmada, one of the signatories of the Proclamation, executed after the Easter Rising.
“Even though we have been talking about partition for the last 100 years in this country, the reality is that the necessary analysis of how it will be put together has not been done.
"We, who want a united Ireland, must do it by the powers of persuasion. It is for us to make the case, as the Scottish National Party are doing in Scotland, " he said.
“It is a challenge that we need to take on rather than moaning about unionism. We have to convince those people who vote for them.”
Mr Ahern said many questions needed to be answered.
What governance structure and electoral arrangements would apply? What would the cost implications be? How would those costs be met?
What changes would be required as far as legal regimes, security arrangements, policing and immigration services are concerned?
What provisions would be needed to put in place to ensure the rights of the new minority in a united Ireland, the unionists, would be respected, he said.
“There are some academic studies going on but, as of now, we haven’t done that work,” he said.
“Unless we undertake the substantial task of analysis and consensus, we will never get to the point where an informed opinion can be made as to whether a Border poll can take place.
“The basis of the Good Friday Agreement is the principle of consent that there will be on another day a chance for people to vote on a united Ireland, but we have to work out what that means and we haven’t done that yet.”
Mr Ahern said unionists were not engaging with the arguments around a united Ireland because they believe that even a recognition of that debate would concede the validity of the principle of a united Ireland.
He said they have not made the case for the union yet as Gordon Brown had done for the union with Scotland before the 2014 Scottish independence poll.
That may yet happen.
“We are going to see a far more united unionist message than appears at the moment,” he said.
Speaking on the War of Independence, Mr Ahern said it showed it was possible to have a sovereign, Irish State even if it was not the 32-county republic many had envisaged.
They had believed the Boundary Commission would be a "stepping stone" towards Irish unity.
He said if there had not been a Civil War, there would have been successes out of the Boundary Commission which would have transferred territory to the Irish Free State.
Former minister for justice Michael McDowell said the Provisional IRA had attempted to coerce the people of Northern Ireland into a united Ireland.
He said Éamon de Valera had never sought to coerce unionists into joining the Irish State.
He said the next census was likely to see close parity between the number of nationalists and unionists and this could provide the basis for a Border poll by consent.