Businessman Denis O'Brien wants the Dáil and State to pay a portion of the costs of his failed High Court action over statements made by two TDs in the Dáil about his banking affairs.
The application has been strongly opposed by lawyers for the Dáil and State. The costs of the seven-day case are estimated at around €1m.
Ms Justice Una Ní Raifeartaigh has reserved her decision on the costs application.
Last week, she dismissed Mr O'Brien's case ruling the Constitution did not allow the court to intervene in relation to utterances in the Dáil.
She found that what was sought by Mr O'Brien was very far reaching and would have a chilling effect on parliamentary speech into the future.
Sara Moorhead SC, on behalf of the Dáil, told the court Mr O'Brien was a very wealthy man.
She said her side was funded by the taxpayer and no one who sued, seeking to have a "chilling effect" on parliamentary speech should get the costs of such as case.
Ms Moorhead and lawyers for the State said they should get their costs against Mr O'Brien.
Maurice Collins SC, for the State, said there was nothing novel in the case and he said Mr O'Brien had sued out of personal interest and to have the court rebuke TDs, Catherine Murphy and Pearse Doherty.
He said had Mr O'Brien won, the consequences for parliamentary speech would have been dramatic and very, very negative.
Mr Collins said the court found the parts of the constitution which give immunity to what is said in parliament, mean what they say, irrespective of the "devices" advanced on behalf of Mr O'Brien.
He said Mr O'Brien's case was aimed at punishing the TDs.
He urged Ms Justice Ní Raifeartaigh to enforce the ordinary rule on costs - where the losing party pays.
Lawyers for Mr O'Brien told the court the case was important, and "undoubtedly novel".
Michael Cush SC said there was no previous case where a citizen with a court order, complained that what was said in the Dáil rendered that order pointless.
Mr Cush said the court had also clarified an area of possible ambiguity concerning the application of Article 15 of the constitution.
He said this was a novel and important case not brought by his client entirely out of personal interest but also for other citizens who wished to protect the privacy of personal information protected by court orders.
There was of course a personal interest but no personal advantage to Mr O'Brien, he said.
