The action relates to an investigation concerning travel and subsistence claims made by Mr Ó Domhnaill between 2006 and 2007, when he was a member of Donegal County Council.
In his judicial review proceedings, Mr Ó Domhnaill claims the commission is not entitled to deal with the matters on grounds that they arose from an anonymous complaint by a member of the public.
Alternatively, he wants orders requiring the case to be heard by a commission comprising members who are bilingual. Otherwise, he said, his rights as an Irish speaker will be infringed.
In opposing the case, the commission and State contend the complaint being investigated relates to alleged duplication of expenses claims and the investigation is in the public interest.
The commission says the complaint was made in a letter dated May 28, 2012, sent to it by the mayor and county manager of Donegal County Council following an investigation by the two men of a referral made to them by the ethics registrar of the council, the commission claims.