Businessman Denis O'Brien is seeking findings that would be "entirely destructive of" and would "effectively undermine" parliamentary speech, the Supreme Court has heard.

Maurice Collins SC, for the State, said Mr O'Brien has brought proceedings arising out of utterances in the Dail about his banking affairs by TDs Pease Doherty and Catherine Murphy that would indirectly substitute the courts instead of the Dáil as being the arbiter of what was "appropriate or inappropriate" parliamentary speech.

Lawyers for the businessman rejected the claims.

Mr Collins says that the proceedings were brought over the manner the Dáil Committee on Procedure and Privilege handled Mr O'Brien's complaint about the TD's statements to the House in May-June 2015.

The CPP dismissed the complaint, which the businessman has challenged in judicial review proceedings.

In his submissions to the Court, Mr Collins said the CPP is an agent of and made up of members of the Oireachtas and under sections of Article 15 of the Irish Constitution, which allows free speech in the Oireachtas, any decision the Committee makes cannot be challenged before the courts.

Counsel said all decisions by the CPP are "non-justiciable" and any attempt to have its findings reviewed by a court breaches the separation of powers. 

Counsel said the State's argument was 'a fortiori" or even stronger than the Supreme Court's 2014 judgment in favour of a Seanad committee against former Senator Ivor Callely over his expenses.

The court ruled the Seanad Committee on Members' Interests was entitled to conclude Mr Callely had in 2008 misrepresented his normal place of residence as being in Bantry, Cork, rather than Dublin when claiming expenses.

That appeal arose out of a 2011 High Court finding that there had been a breach of fair procedures when the Seanad Committee decided in July 2010 that he had deliberately misrepresented his place of residence.

Earlier in his submissions to the court, Michael Collins SC, for the CPP, said any finding that the committee's decisions could be the subject of court actions meant that any protection it has under Article 15 was "wholly illusionary".

Counsel also said by bringing proceedings against over the committee's decision Mr O'Brien was seeking to have the two TD's sanctioned for their statements,

Both lawyers were making submissions on the second day of an appeal brought by the businessman against the High Court's dismissal of his case against the Oireachtas and the State over the statements made by the two TDs.

In reply to the State and the CPP's submissions, Eileen Barrington SC, for Mr O'Brien, said the State respondents were making "a stark case" which her client, who was directly affected by "an abuse of privilege" rejects.

Mr O'Brien claims the CPP failed to obey its own rules when dismissing his complaints about statements.

As a result, the courts should step in to ensure vindication of the rights of a non-member of the Oireachtas, he claims.

He also claims there was no evidence the TDs acted in good faith and responsibly when they made the statements.

In his action, Mr O'Brien claims statements made by the two deputies after he secured injunctions against RTÉ restraining it publishing details about his banking affairs with IBRC was an unwarranted interference in the judicial domain.

This was because the statements revealed most of the information subject of the injunction, which made the court proceedings pointless.

In 2016 Ms Justice Una Ni Raifeartaigh dismissed Mr O'Brien's action, stating what he sought was not allowed due to the separation of powers and would have a chilling effect on parliamentary speech.

His appeal centres on the finding by the CPP that the TDs had breached relevant standing orders on how it addressed Mr O'Brien's complaint.

Following the conclusion of submissions from both sides on Tuesday evening the seven-judge Supreme Court reserved its decision.

The Chief Justice Mr Frank Clarke said it may take some time before the court was in a position to deliver its judgment.