Mary O'Rourke and Brian Cowen at a FF press conference.
She said he should have resigned after his infamous "congested" radio interview.
Ms O'Rourke backed up the attack on Mr Cowen by her nephew, former Finance Minister Brian Lenihan.
Mr Lenihan said he was "disappointed" by Mr Cowen's performance and he had to provide the leadership when the Taoiseach didn't.
Ms O'Rourke told the Irish Independent that Mr Cowen should have stepped down after the now infamous autumn radio interview after which he was accused of being hungover on live radio.
"He should have gone after the radio appearance but he was a good leader up to a point," Ms O'Rourke said.
"That episode was pivotal in the wrong way for the (Fianna Fail) party -- having said that, I wish him well."
Mr Cowen came in for criticism in the aftermath of the interview, which was dubbed "garglegate".
The interview, on RTE's 'Morning Ireland', took place after a night of revelry with his Fianna Fail colleagues at Galway's Ardilaun Hotel in September last year.
During the interview Mr Cowen mistakenly referred to the Croke Park Agreement as the Good Friday Agreement. The appearance sparked a wave of criticism from opposition TDs with Michael Noonan, the then Fine Gael spokesman on finance, and now minister at that department, claiming there were concerns over the leadership of the country.
Ms O'Rourke added that Mr Cowen seemed to have a problem communicating with the public.
"It did seem to get to a point where it seemed he didn't want to engage with the public." And she sharply criticised Mr Cowen's failed attempt to appoint six new ministers in the dying days of his rule at the beginning of this year despite warnings from the then coalition partners in the Green Party.
"It was extraordinary when he proceeded to try to put people into those jobs -- it was hubris," said Ms O'Rourke.
She was speaking after she opened a conference on older people's consumer rights.
She also said that recent analysis of the new Cabinet focused too much on the age of the ministers, arguing that experience was necessary in Ireland's negotiations in Brussels over the bailout terms.
"I would much prefer to see the wise old head of Michael Noonan negotiating for us in Europe rather than some pesky 34-year-old," she added.
"There is far too much emphasis on the average age of the cabinet being 55; it's experience that matters most these days."
Efforts to contact Mr Cowen were unsuccessful last night.
sam smyth, page 39