The Standards in Public Office Commission has confirmed that it is to formally investigate allegations relating to travel and subsistence claims by Fianna Fáil Senator Brian Ó Domhnaill.
Upon receipt of the allegations originally earlier this year, SIPO appointed an inquiry officer to look at the matter.
The commission considered that report and has now decided to initiate a full investigation.
These types of investigations are normally held in public. There is no date set yet for the hearing to take place.
It is understood the claims relate to 2006 and 2007 when Senator O Domhnaill was a member of Donegal County Council.
Its expected the Commission will hold a public hearing into the matter in due course.