A legal challenge by one of Ireland’s richest men risks destroying parliamentary speech, a lawyer has said.

The action taken by telecoms billionaire Denis O’Brien after two politicians disclosed details of his banking affairs is not the business of even the Republic’s highest court, barrister Michael Collins added.

Mr O’Brien appealed to Ireland’s Supreme Court after a failed attempt to sue the legislature following statements made by two TDs (MPs) three years ago which rendered his previous court injunction almost pointless.

The lawyer for the Oireachtas parliament, senior counsel Mr Collins, said the court should not scrutinise its elected members’ comments.

He added: “That is ultimately destructive of parliamentary speech because unless you can speak knowing that you are not going to be subject to some form of oversight other than the oversight provided for in the constitution, that you can be made answerable to a court – even if indirectly – then (free speech) … is effectively undermined.”

He said it was not judges’ role to scrutinise speech in the Oireachtas in Dublin based on how bad, incompetent or erroneous it was.

“This appeal avowedly seeks the court to circumscribe parliamentary speech and involves the substitution of the court, and this court as the final court in this jurisdiction, as the ultimate arbiter of parliamentary speech in this jurisdiction.”

Parliamentary speech is privileged in Ireland under the country’s constitution and the separation of powers between lawmakers and the judiciary is closely guarded.

The High Court decided it had no power to rule on issues raised by Mr O’Brien concerning how the Oireachtas Committee on Procedure and Privileges dealt with the statements about his financial matters and his complaints.

Media tycoon Mr O’Brien is appealing against that decision but did not attend the hearing before seven judges in Dublin on Tuesday.

Mr Collins added: “To get the court to accede to the application here it would inevitably be drawn into a consideration of what occurred on the floor of the House, which I say simply stands completely outside the function of this court.”

Mr O’Brien had obtained an injunction in 2015 preventing Irish broadcaster RTE from reporting on what he claimed were stolen files of his banking records with the state-owned IBRC, formerly rogue lender Anglo Irish Bank.

He said Independent TD Catherine Murphy and Sinn Fein TD Pearse Doherty knew exactly what they were doing when they later read details of the file into the public record.

But a High Court judge upheld parliamentary freedom of speech and the separation of state powers.