Taoiseach Brian Cowen has asked Fianna Fáil Senator Ivor Callely to provide a written explanation of his expenses claims.
Mr Callely, who retains his Dublin home and constituency office in Clontarf, Dublin, claimed just over €81,000 in expenses from December 2007. His home on Sheep’s Head peninsula in West Cork is 370km from Dublin.
Mr Cowen has not spoken to Mr Callely about the matter but has asked Fianna Fail general secretary Sean Dorgan to instruct the Dublin senator to furnish a full explanation in writing "that will clarify the issue" to the Cathaoirleach of the Seanad Pat Moylan.
In a statement, Fianna Fail said the Taoiseach was concerned that all members of both Houses of the Oireachtas should fully comply with the rules. Details of Mr Callely's expenses emerged in a weekend newspaper report.
TDs and Senators resident within 24km (15 miles) of Leinster House receive a daily allowance of €55, reduced from €61.53 in August 2009.
Those living more than 24km away can choose either a flat daily allowance or an overnight allowance and separate travel allowance. The overnight allowance is €126, reduced last year from €139.67.
Official travel rates since March last year are €0.59 a km up to 6,437km and €0.2846 a km after that.
In March this year he returned an expenses cheque for €3,987 and said he wished his expenses and allowances to reflect his actual expenses. "I was hoping that the new allowance system would accommodate my situation, where my appointment to the Seanad was from my Kilcrohane abode, but I do also reside in my Dublin abode, the new system only accommodates one address."
The Oireachtas had written to him a number of times and in one letter in 2008 the members' services section sought to clarify "for the avoidance of doubt and for absolute certainty for future audit purposes" that his West Cork address was his "normal place of residence for the time being".
Mr Callely was appointed a Senator in August 2007, as a Taoiseach's nominee, after he lost his Dáil seat and failed to win a seat in the Seanad election.
That year in correspondence with the Oireachtas he said "my personal situation has changed since June 2007 and while I retain my Dublin home and my constituency office my current principal residence is Kilcrohane, Bantry, Co Cork
In 2006 he was forced to resign as minister of state for transport, when it emerged that a builder had painted his house for nothing and he had not disclosed this to the Standards in Public Office Commission.