Businessman Denis O'Brien has lost his High Court action over statements made by two TDs in the Dáil about his banking affairs.
Ms Justice Úna Ní Raifeartaigh dismissed the telecoms and media tycoon's action against the Clerk of the Dáil and the State after finding the Constitution prohibits the courts from intervening over speeches made in the Oireachtas.
In his action, Mr O'Brien alleged Social Democrats TD Catherine Murphy and Sinn Féin TD Pearse Doherty "clearly disregarded" the constitutional separation of powers between parliament and the courts when they made statements about his banking affairs in the Dáil in May and June 2015.
Mr O'Brien alleged their intervention amounted to "unwarranted interference" in the judicial domain. He also argued the Dáil Committee on Procedure and Privileges, which rejected his complaints about the conduct of the politicians, failed to "properly police" the TDs.
Mr O'Brien asked the court to censure both TDs and lay down a line beyond which deputies cannot pass.
But the respondents successfully argued the court could not intervene as Article 15.13 of the Constitution means members of the Oireachtas are not amenable to any authority other than the Oireachtas for "utterances" in the Dáil or Seanad. They insisted this conferred an absolute immunity over what is said in the Oireachtas.
In her judgment, Ms Justice Ní Raifeartaigh said the case raised important issues.
What Mr O'Brien was seeking would have "very far-reaching" effects, she said. The courts "simply do not have a role in policing parliamentary utterances" except perhaps in grave exceptional circumstances of which this case was not one.
Ms Justice Ni Raifeartaigh said such a review could consider issues such as whether and when a Dáil deputy may discuss matters which are before the courts, and in particular reveal matters that are the subject of an injunction.
Ms Murphy welcomed the ruling and said she had only ever acted in the public interest.
In a statement, Mr O'Brien said he was "disappointed" and would take time to consider whether or not to appeal.