The High Court cannot grant the declarations sought by businessman Denis O'Brien, lawyers for a Dáil Committee have claimed.

The Court was told the only authority with the power to investigate what is said in the Oireachtas, is the Oireachtas itself.

The lawyers were making submissions in the legal action by businessman Denis O'Brien against the committee, members of the Dáil, and the State about statements made in the Dáil by two TDs in May and June last year.

Mr O'Brien said Social Democrats TD Catherine Murphy, and Sinn Féin TD Pearse Doherty, effectively decided the outcome of a court case he was taking against RTÉ in their statements in the Dáil.

He wants the High Court to express its disapproval of what they did and how the committee dealt with it, by making declarations.

Dáil deputies had to have certainty about what they could speak about in the house, lawyers for the Oireachtas told the High Court.

They could not have that if they were constantly under the shadow of possible scrutiny from the courts or another authority.

Senior Counsel Michael Collins said one could see appalling consequences if a court engaged in scrutinising Dáil debates before they happened or rebuking deputies afterwards.

Yesterday, Mr O'Brien said he wanted to ensure that no other citizen would have their confidential banking details revealed in the Dáil, while a court case was under way. He said he wanted TDs to respect the High Court.

Senior Counsel Michael Collins told the court this morning that not alone did the Constitution say that the courts could not deal with complaints made about matters said in the Dáil, it also set out that the Houses of the Oireachtas were the only designated authority with exclusive power to deal with such complaints.

Mr Collins said Mr O'Brien wanted the High Court to make declarations. He said those declarations could not be made.

He said if the declarations had no legal effect, then they should not be made as it was not the function of the court to make such declarations.

If they did have a legal effect or consequence, Mr Collins said, they were impermissible under the Constitution.

He said Mr O'Brien was asking the court to ascribe impropriety to the statements made by Deputies Murphy and Doherty in the Dáil and he said the court could not do that.

Mr Collins said Dáil deputies had to have certainty about what they can speak about. If they were constantly under the shadow of possible scrutiny from the courts or other authorities then they could never have certainty about what they could say in the Dáil.

He said the protection given by the Constitution to utterances by members of the Oireachtas in the House was not fuzzy or imprecise, it was sharply defined.

He said case law was full of statements about the exclusive power of the Dáil to deal with any questions that arose about any debate in the house.

Yesterday, Mr Collins said the High Court had no power to intervene in the process of freedom of speech of Dáil deputies on the floor of the House.

He said Article 15.13 of the Constitution expressly prevented the courts from interfering in what members of the Oireachtas say on the floors of either house.

He said the circumstances of this case were not so exceptional as to allow the court to cross the line between the judiciary and another branch of government.

He said the TDs had not "determined" Mr O'Brien's case against RTÉ, as claimed by Mr O'Brien, as the exercise of free speech by Dáil deputies could never amount to the exercise of a judicial function.

Mr Collins said the court did not have the power to intervene in the internal workings of a Dáil committee.

The committee did not make Mr O'Brien subject to its processes and was not seeking to infringe on his rights.

It was conducting an internal inquiry into its own affairs and Mr O'Brien had no right to have the inquiry "done properly".

Mr Collins said the idea that deputies would be free to say what they want on the floor of the house was a critical and integral part of the separation of powers.

He said any of the remedies sought by Mr O'Brien required the High Court to examine statements made by Dáil deputies and their effect on Mr O'Brien and this was something the court could not do.

He said politicians were entitled to speak freely in parliament without the "chilling hand" of litigation resting on their shoulders.

The case will continue on Tuesday.