The Standards in Public Office Commission has defended the cost of investigating a Fianna Fáil Senator's expenses insisting the majority of the money was spent on challenging court cases taken.

Freedom of Information documents released to the Journal.ie this morning showed it has cost €300,000 to carry out an investigation into the alleged duplication of expenses by Brian Ó Domhnaill.

Mr Ó Domhnaill is accused of claiming expenses for different events held at the same time.

Sipo has insisted the majority of the €300,000 was spent on challenging court cases taken by the Donegal Senator.

Mr Ó Domhnaill took cases in the High Court and the Court of Appeal seeking to prevent Sipo investigating the duplication of expenses. He lost both cases.

A Sipo spokesman said the body had won its claim for costs so the majority of the monies would be returned to the exchequer.

A total of €25,000 was spent on translation services, which was a request by Mr Ó Domhnaill. He sought to conduct the public hearing in Irish.

A Sipo spokesman said this was his constitutional right but it did add significant costs.

The spokesman said a standard investigation would cost a fraction of Mr O’Dómhnaill’s.

“We have a duty to investigate all complaints forwarded to us. The majority of the costs involved here related to Mr Ó Domhnaill’s decision to take a High Court challenge and to subsequently refer it to the Court of Appeal.

“That is his right and entitlement. However it should be stressed that a large sum of our legal costs will be refunded to the exchequer.”

Sipo has made a preliminary decision in the case and a final report is being drafted at present.

Mr Ó Domhnaill declined to comment until the office has concluded its work.