Billionaire Denis O'Brien has had his case over statements made in the Dail about his banking affairs dismissed by the Supreme Court.
The court, sitting in Galway, delivered a unanimous judgment rejecting Mr O'Brien's appeal against the High Court's decision to dismiss his action.
The High Court action arose from statements made by Social Democrats TD Catherine Murphy and Sinn Fein's Pearse Doherty in a debate in 2015.
Mr O'Brien claimed the statements, made after he won an injunction against RTE to prevent it broadcasting details of his banking affairs, interfered with the court decision and damaged his constitutional rights.
The Dail's Committee on Procedure and Privileges (CPP) rejected his complaint, saying the TDs did not breach the rules.
However, Mr O'Brien took the case to the High Court, saying the manner in which his complaint was dealt with breached the committee's own rules.
Yesterday, the Supreme Court ruled interference by the court in the case would be "impermissible under the Constitution".
Mr Justice Frank Clarke said: "Mr O'Brien sought relief in respect of two different types of claim.
"The first concerns directly statements or utterances made by two deputies in the Dail. The second concerned the manner in which the CPP dealt with a complaint he had made in respect of those utterances.
"In its judgment, this court identified two significant barriers to the justiciability of issues arising from a set of restrictions derived from the actual wording of the relevance of sub-articles of Article 15 of the Constitution which confer significant privileges and immunities on the Houses of the Oireachtas.
"The court has concluded the challenge which Mr O'Brien has sought to bring to the decision of the CPP involves, in substance, an indirect or collateral challenge to the utterances of the deputies themselves.
"The only practical consequence of a successful outcome to proceedings such as this would be that it might lead to a reconsideration by the CPP of its decision in respect of Mr O'Brien's complaint.
"If that should lead to a different result, then a court would have been at least indirectly or collaterally involved in dealing with utterances made in the Houses. In the court's view, such course of action is impermissible under the Constitution."
Mr Doherty tweeted: "Over the moon as Supreme Court dismisses Denis O'Brien's appeal about comments made in the Dail by myself and Catherine Murphy. Common sense prevails."
Ms Murphy tweeted: "Obviously delighted with today's Supreme Court ruling which reinforces decision of both CPP and High Court, that myself and Pearse Doherty did not abuse parliamentary privilege."
The decision comes less than a week after Mr O'Brien lost a High Court libel action against the Sunday Business Post.
Speaking outside Leinster House, Mr Doherty said it was a "good day" for parliamentary democracy.
"I shudder to think if both these judgments had gone the other direction because there is no doubt that there would be a huge chill effect on parliamentary debate and in relation to press coverage and reporting of important matters," he said.