Mr Campbell said Mr Ahern deserves significant credit, along with Mr Blair, for creating the opportunity for peace in the North.
“I will always defend Bertie, and he is getting vilified now, but I will always defend Bertie because he took big risks,” Mr Campbell said of Mr Ahern, who resigned as taoiseach in May 2008.
“He took a lot of abuse; he took a lot of shit along the way, but he took big risks.”
Asked whether he felt Gerry Adams was a member of the IRA, Mr Campbell said he had always viewed Sinn Féin and the IRA as two parts of the same organisation.
“He is a leading figure and we always used to say they were two sides of the same coin,” he said.
Mr Campbell said he is often criticised for “being too close” to the Sinn Féin leadership, which stems from his admiration of Martin McGuinness, despite his admitted role in the IRA.
“I have always felt that Martin McGuinness has been an immense force for change,” he said.
Mr Campbell added that ultimately a decision had to be taken to deal with people like Mr Adams and Mr McGuinness to try and achieve a permanent peace.
“I am of the opinion, we had to deal with all sorts of people, and about Gerry Adams, Tony Blair and others had to make a judgement as to whether he had truly changed, and truly decided that there was a democratic path and he decided to go down it,” he said.
Mr Campbell, who was in Dublin to promote his book, Winners, was deeply critical of supporters of British Labour leader Jeremy Corbyn, whom he accused of zealously treating him like a “messiah”.