Labour and Fine Gael have said that they intend to raise what they say are new concerns about Mr Ahern's tax affairs when the Dáil returns next month.
Speaking on RTÉ Radio's Morning Ireland, Mr Gilmore said Mr Ahern's credibility was being undermined and his authority was being eroded by each new revelation about his finances.
Mr Ahern is facing increasing Opposition pressure over his performances at the Mahon Tribunal.
He is due to give evidence again in January to discuss new issues in relation to £5,000 lodged into his building society account in January 1994.
He last appeared before the inquiry for two consecutive days before Christmas, and over four days in September, when his 1990s personal finances were investigated.
Tribunal lawyers focused part of their inquiries on cash lodgements made to Mr Ahern during a 13-month period between 1994 and 1995 while he was Minister for Finance.
