'It's a lot of trouble for SDLP... They were hoping to hold the ground in Derry but that didn't happen'
Mr Ahern said he believes a referendum on unity should not be considered until all the preparations have taken place and until the “work is done”.
He described the Northern Assembly election as "a great day" for Sinn Féin, in the wake of calls by the party's leader for a border poll in the next decade.
Counting resumed in Northern Ireland Assembly election this morning with about two-thirds of the 90 seats currently filled.
"[Sinn Féin] are in pole position. I think nobody predicted that they'd hold all their seats, not to mind that go up, so it's a big day for them, there's no doubt about that,” he told The Anton Savage Show on Newstalk.
"It's a lot of trouble for SDLP. They were hoping to hold the ground in Derry but that didn't happen. After reorganising themselves, Sinn Féin did well again."
This comes as Mary Lou McDonald said she believes that a referendum on the reunification of Ireland could take place within the next five years.
She was speaking as Sinn Féin is poised to become the biggest party in Northern Ireland with a nationalist First Minister in Stormont for the first time.
Ms McDonald said partition has been “disastrous” and that there will be constitutional change on the island of Ireland within the next decade.
When asked if McDonald was right in the prediction of a border poll in the next decade, Mr Ahern said: “In the election in the North, there was very little talk by nationalist parties about a border poll or about unity.”
“The election was very much about what we know as the economic bread and butter issues of the day.
"The international media are perplexed with the idea of unification but on the ground, it was not the issue. They weren't talking about the protocol, and they weren't talking about a border poll either. Of course, it's continually on the agenda, it's on the agenda since 1998. The provisions for having a border poll [are] there.
"Sinn Féin in recent years have moved their position to a position which I've been articulating for a long time, that you don't have that poll until the work is done.
"I agree with them that some of the preparatory work is underway academically at the moment... but until you work out what does unity mean, what kind of a new Ireland is it, how would you bring it all together? There's no point having an election until you do that."
He added: "How is this going to be funded? 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."
Mr Ahern said now Fianna Fáil has a “big challenge” to increase popularity in the polls.
"I used to nearly lose sleep when we went under 40pc and it's not that long ago. Now people seem to celebrate when we go a bit over 20pc,” he said.
“There's a big challenge. I think organisationally-wise across the 40 constituencies FF need to be reenacting their ability to organise themselves, attract candidates and have policies.
“As it stands now, Sinn Féin are in a very strong organisational position right across the Republic. They don't dominate everywhere, but they are on 30pc of the vote in polls now for the last two years. I think it's not inevitable that they're going to go to 35pc. There's a big job to be done."