The Seanad Committee on Members' Interests is to begin an investigation into mobile phone expenses submitted by Senator Ivor Callely in November 2007.
The decision came after a meeting lasting over six hours.
Read the statement from Seanad Committee
The committee will ask Senator Callely to provide a written statement in relation to the mobile phone expenses by 23 August.
Senator Callely has already acknowledged the claims should not have been made and he has written to the Leinster House authorities to repay the sums involved, just under €3000.
The committee is also investigating separate complaints made against senators Larry Butler, Rónán Mullen and Ann Ormonde.
In a statement this evening, Senator Mullen said he is seeking an urgent meeting with the chairman of the committee following what he called the 'unspecific nature' of its statement.
He claimed it failed to specify that the complaint made against him did not allege any impropriety on his part in the claiming of expenses.
It was also confirmed tonight after a two-hour meeting, that a sub-committee of the Fianna Fáil National Executive will also proceed with an inquiry into the Senator's expenses.
It is expected he will be invited to appear before the sub-committee within the next few weeks.
The report of that body is likely to be completed by early September.
Senator Callely has already been temporarily suspended from the Fianna Fáil organisation pending the outcome of the party's inquiry.
Earlier, Minister for Agriculture Brendan Smith said Fianna Fáil wants questions relating to the most recent allegations about Mr Callely's expenses cleared up as soon as possible.
Speaking on RTÉ Radio, Mr Smith said the matter needed to be dealt with urgently.
Members of the Seanad committee examined a number of complaints concerning the mobile phone expenses claim, including one from Green Party TD Paul Gogarty.
Senator Callely submitted four receipts for mobile phone expenses in November 2007 when he was a TD covering the period January 2002 - July 2006.
It has since emerged the receipts came from a company that had ceased to trade.
Senator Callely issued a statement over the weekend relating to the claims.
The Dublin senator said he submitted four receipts for mobile phone expenses 'in good faith', but acknowledged the claims should not have been made.
Mr Callely said he did not know how these receipts were issued by a company that had ceased trading and had no reason at the time to believe that they were not in order.
He said he had now refunded a total of €2,879.45 to Leinster House.
Last month, Senator Callely was suspended from the Seanad for 20 days after members found he had misrepresented his normal place of residence as west Cork instead of Dublin for expense purposes.
Speaking in Tullamore in Co Offaly yesterday, Taoiseach Brian Cowen said Senator Callely's statement raised more questions than it answered.