The Taoiseach has requested a written explanation from Senator Ivor Callely about his expense claims.
Senator Callely had been urged to make a public statement on his expenses after the relevation that he claimed mileage and subsistence allowances from a house in Cork, rather than his base in Dublin.
The request was delivered through the General Secretary of Fianna Fáil.
Mr Cowen asked for the explanation to be delivered to the Cathaoirleach of the Seanad, who chairs the Committee on Procedure and Privileges for the Upper House.
It comes as TDs and Senators have had details of their expenses published online.
The move follows the introduction of a new system that links attendance records to the amount of expenses a member can claim.
The body responsible for running the Houses of the Oireachtas said the move will result in a greater level of accountability.
Exact figures on attendance records will not be published until later in the year.
The move comes as the latest attendance figures for March were obtained by RTÉ News under the Freedom of Information Act.
They show that the Dáil officially sat for 11 days during the month and that the majority of TDs did clock in for that amount.
The TDs with the highest attendance records were each counted in for 20 days.
They were Noel O'Flynn, Michael Mulcahy, John McGuinness and Niall Collins of Fianna Fáil; Labour's Pat Rabbitte and Tommy Broughan, Fine Gael's Olivia Mitchell and Independent TD Finian McGrath.
Fianna Fáil's Dr Jim McDaid clocked in on six days, while former Defence Minister Willie O'Dea clocked in on four days.
Sheehan defends expenses
The figures show that Fine Gael TD PJ Sheehan received the largest amount in Oireachtas expenses.
Mr Sheehan said somebody has to come on top of the list and under this scheme the person with the greatest distance to travel has come out on top.
Living close to Mizen Head in west Cork, Mr Sheehan is the only TD in Band 12, covering those based 360km or more from Leinster House.
He welcomed the publication of the expenses details, saying Fine Gael had long called for greater transparency.
Mr Sheehan added that the cost of running his full-time constituency office in Bantry is more than he can claim under the vouched and audited expenses.
Fianna Fáil Senator Marc McSharry welcomed the publication of expenses on the Oireachtas website and said it should always have been the case.
He said no member should be immune to the aspiration and the practice of the highest levels of propriety with regard to expenses.