Former taoiseach Bertie Ahern has warned that there is a lot of "preparatory work" that should be done before a potential Border poll.
He said failure to do this could lead to a repeat of the failed Scottish independence referendum.
With Sinn Féin set to become the biggest party in Northern Ireland as counting in the Assembly election continues, talk will inevitably turn to a United Ireland.
Party leader Mary Lou McDonald has suggested a Border poll could take place within the next five years.
Mr Ahern, who was heavily involved in the 1998 Good Friday Agreement, warned Sinn Féin that people in the North will not vote for a United Ireland "if they don't see the facts".
In an interview with Newstalk, Mr Ahern said it has been a good election for the party, however, he said they should now look into the practicalities of a Border poll.
How is this going to be funded?
"How is this going to be funded?” Mr Ahern asked on The Anton Savage Show.
“How is the administration going to work? Where are the practical details? Where are the position papers?
“In practice none of this has been thought through. Even though we've been yapping about it for years, it hasn't been done.
“Until that work is done, the idea of having a border poll - whether that takes two years, five years, I'm not going to say when - it has to be done.
“Otherwise you're asking people to vote, and do you know what will happen when you do that, they'll just vote no, if they don't see the facts, and they won't vote."