Businessman Denis O'Brien has lost his appeal over statements made in the Dáil about his banking affairs.
Mr O'Brien appealed the High Court finding that the courts had no power to intervene on issues raised by him about how the Dáil's Committee on Procedure and Privileges dealt with the statements and his complaints.
But the court found his claim amounted to an indirect or collateral attack on utterances made in the Dáil and that was impermissible under the Constitution.
Mr O'Brien was granted an injunction against RTÉ in April 2015, in relation to a proposed broadcast about his banking affairs.
He claimed Social Democrats TD Catherine Murphy and Sinn Féin TD Pearse Doherty then revealed information on the floor of the Dáil, which made the injunction pointless.
The Dáil Committee on Procedure and Privileges found the TDs had not breached the relevant rules relating to discussing matters that were before a court.
It found Ms Murphy acted in a responsible manner, in good faith and as part of the legislative process.
Mr O'Brien argued the committee's finding was based on an error in interpreting the relevant Dáil standing order.
He claimed there was no evidence the TDs were acting in good faith.
The High Court ruled the courts could not intervene in relation to utterances made in the Dáil or how the CPP dealt with them.
Mr O'Brien challenged the High Court ruling in relation to the committee. He said the committee breached its own rules and the courts could step in.
But this morning, sitting in Galway, the Chief Justice, Mr Justice Frank Clarke, said the Supreme Court had dismissed the appeal.
In a statement this afternoon, Mr O'Brien said he was "disappointed" by the decision but respected the court's ruling.
The Supreme Court judgment in full
Mr Justice Clarke said the court had decided last week in the Angela Kerins case that the courts could review the actions of a house of the Oireachtas or an Oireachtas committee to determine the lawfulness of the actions concerned and could intervene where a committee acted unlawfully, outside its remit.
However, he said there was a significant limitation on the circumstances in which the courts could do that.
He said the High Court had noted in Mr O'Brien's case that any undue interference by the courts would have a "chilling" effect on free speech in the Oireachtas and its legitimate constitutional business.
He said this judgment confirmed that matters relating indirectly or collaterally to utterances made in the Houses of the Oireachtas could not be reviewed by the courts.
The only practical consequence of a ruling in Mr O'Brien's favour would be a reconsideration by the CPP of its decision in Mr O'Brien's complaint.
If that led to a different result, he said a court would have been at least indirectly or collaterally involved in dealing with utterances in the Oireachtas.
And such a course of action was impermissible under the Constitution.
The court found that if there was any exception to this rule there would have to be an egregious breach by the Oireachtas of its obligation to protect the rights of citizens or a persistent failure to deal with matters.
It found neither situation applied in this case.
Mr Justice Clarke said there may be an argument that the CPP was in error in some of the ways it dealt with Mr O'Brien's complaint.
He said the committee was correct to find it had no role in determining if someone was in contempt of court.
But he said it did not address whether it was appropriate for deputies to act in a way which may have deprived a citizen of the benefit of a court order.
He said it might also be said the CPP over-readily came to the view, without sufficient inquiry, that those concerned had acted in good faith.
But even if it could be said the CPP reached an impermissible conclusion, that would fall a very long way short of the truly exceptional failures, in which a court could intervene.
Mr O'Brien also appealed the High Court's ruling that the case was not novel enough to depart from the usual rule that the losing party pays the costs.
The court allowed his legal team time to study the judgment and will address that issue on another date.
This judgment was due to have been delivered last week, but was postponed following a request from Mr O'Brien's lawyers who were concerned it could affect his defamation action against the Sunday Business Post.
Mr O'Brien lost that case last Friday when a jury found the articles complained of were not defamatory.
O'Brien to consider implications of ruling
In his statement this afternoon, Mr O'Brien thanked the court for postponing its decision from last week and said he would consider the judgement and its implications.
He said: "A consequence of this decision would seem to be that an elected representative can make any allegation about any citizen in Dáil Éireann - be it true or untrue, defamatory or otherwise, without sanction. I fear that this leaves Dáil privilege open to further abuse in the future.
"It was acknowledged in the High Court Judgment in this case that the law concerning parliamentary privilege and the regulation of members' conduct in the Houses of the Oireachtas warrants a review.
"I appreciate the Courts' views on the important issues raised in this action and I hope that the legislature carefully considers the clear suggestions for reform in this area, as counselled by the High Court.
"I am satisfied that it was right for me to ask the Court to rule on these important issues."
Speaking after the judgment, Ms Murphy said it was yet more recognition by the courts that parliamentary privilege is an important aspect of our system of democracy.
In a statement, she said: "Whilst I am pleased that the courts have recognised the constitutional protections afforded to those of us who may find ourselves in the position of having to use parliamentary privilege for a matter of public interest, it is hugely incumbent on us, as elected representatives, to recognise that such privilege must only be exercised with great responsibility.
"The public interest must always be to the fore and that is what has underpinned everything I have done to date."
Mr Doherty said he was "absolutely delighted" the Supreme Court had dismissed Mr O'Brien's appeal.
Speaking on RTÉ's Today with Sean O'Rourke, Mr Doherty there was always a danger that the Supreme Court ruling could have gone the other way.
Had this happened, he said, it would have had a "huge, chilling effect'"on parliamentary debate and on democracy.
He said: "This judgment clearly shows that parliamentary privilege is sacrosanct and cannot be tested before the courts."
Mr Doherty said there was an onus on elected representatives to use that privilege very selectively, which he said they do.