Brian Cowen and Bertie Ahern are to have thousands of euros added to their six-figure pensions.
The Sunday Times reported that the windfall is due to a provision in the Lansdowne Road agreement on public-sector pay.
The two former Taoisigh currently get around €134,000 per year, but last month's pact will see that rise by
The pension of former Minister for Finance Charlie McCreevy will increase from €108,000 to €110,000.
Renua party leader, Lucinda Creighton, said that taxpayers will find the increases particularly difficult in a week where the Banking Inquiry showed what she called the "serial ineptitude of our leaders".
Mr Cowen told the inquiry that "the primary responsibility of the crisis rests with the banks themselves".
Speaking on RTÉ's 'The Week in Politics', Minister for Public Expenditure and Reform Brendan Howlin defended the increases, saying: "We introduced further pension cuts, on top of the ones introduced by Fianna Fáil, and pay cuts for those over 65,000, in the case of this administration - everybody over 65,000 took a pay cut.
"But that was anchored into emergency legislation."
He added: "If I allowed no change, one day, a challenge would be taken to that and the FEMPI [Financial Emergency Measures in the Public Interest] legislation would collapse and we would be down €2.2bn, which would be ruinous for us."