The Government Chief Whip John Curran has confirmed that a second Senator is being investigated over the expenses row.
Fianna Fail Senator Larry Butler is claiming an extra €20,000 a year in travel allowances from County Carlow, despite having a home and political base in Dublin.
The new revelation comes as pressure mounts on Senator Ivor Callely to resign from the Fianna Fáil party or have the whip removed, while he is under Parliamentary inquiry over his expenses.
It has also raised further questions about the operation of the expenses system for politicians.
Mr Curran said the party is awaiting a written explanation from Senator Butler.