The Supreme Court has declared the actions of the Dáil's Public Accounts Committee as unlawful in its treatment of the former chief executive of the Rehab Group, Angela Kerins.
The court said by conducting a public hearing in a manner significantly outside of its terms of reference and which also departed significantly from the terms of the invitation issued to Ms Kerins, the committee had acted unlawfully.
The Supreme Court also noted that issues raised in these proceedings about the conduct of Dáil committees were capable of being remedied by the Oireachtas.
It is now open to Ms Kerins to return to the High Court to pursue further legal action in relation to the case.
She will be awarded costs against the Dáil in relation to this first part of her High Court case and the costs of the appeal.
Ms Kerins, who was not in court today, has welcomed the judgment and said she would be reviewing it with her legal team.
In a statement, she said: "I am very grateful to the courts on how my case has been dealt with and I am very pleased with the outcome today. I will not be making any further comment on this important judgement by the Supreme Court until I have considered it in detail and reviewed it with my legal advisors."
Chief Justice Mr Justice Frank Clarke said today's judgment was concerned only with two specific questions which followed on from the court's main judgment earlier this year.
The first was whether it would be appropriate to join the Dáil as a respondent in the proceedings in substitution for the individual members of the PAC.
He noted there was no objection to this and the Supreme Court made an order substituting the Dáil as the respondent.
Mr Justice Clarke said the second issue was whether it could be appropriate to characterise the actions of the PAC as having invited Ms Kerins to attend on one basis but acting in a significantly different manner once she attended.
The court was of the view that it was appropriate to characterise the actions as such and therefore Ms Kerins was entitled to a declaration that the PAC had acted unlawfully.
The court also noted that the issues raised in the proceedings were capable of being remedied by the Oireachtas.
If there were genuine grounds for inquiring in the public interest into a particular matter there was no reason why a committee could not seek to have its terms of reference adjusted, the court said.
Likewise, the terms of an invitation to a citizen to attend a hearing could be altered provided it took place in a timely and fair manner.
The court said there was no legal barrier to a citizen answering questions which go beyond the scope of an invitation.
The court also said an appropriate mechanism to provide a remedy in the event of a committee acting inappropriately was a matter which the Oireachtas itself could put in place.
The making of the declaration brings to an end the first "module" of Ms Kerins' case which was before a divisional High Court, the Chief Justice said adding it was now a matter for the parties to consider whether or not any outstanding issues remain and if so this should be argued before the High Court.
Earlier this year, the court found it had the power to make such a declaration, if the committee "as a whole" acted outside its remit and breached the terms of the invitation it extended to Ms Kerins to appear.
In its judgment, in February, the Supreme Court found there was no "absolute barrier" to bringing proceedings concerning the actions of an Oireachtas committee.
It found it had the power to declare the Public Accounts Committee's actions in Ms Kerins' case, unlawful in light of the fact that the committee was acting outside its jurisdiction, and another Dáil committee had come to the same view.
Ms Kerins claimed two hearings in February and April 2014, amounted to a witch hunt against her.
Her lawyers said the invitation issued to Ms Kerins was on limited terms, but other entirely private matters were pursued at the February hearing.
Her Senior Counsel, John Rogers, described what happened as an outrage.
Lawyers for the State said the court had decided the Committee acted beyond its remit.
They said this was a significant and fully accepted finding and the Court should not go any further.
A declaration that the committee acted unlawfully would have a chilling effect on the work of parliamentary committees they said.
The Supreme Court stressed it was dealing with the actions of the Public Accounts Committee as a whole and said the committee was acting "broadly in unison".
The Chair did not seek to prevent any line of questioning and no other member raised any issues of that type, the Chief Justice said.
Quite a number of members of the PAC did seem to concentrate their questions on issues which were at least in general terms within the scope of the invitation but at least three including, significantly, the Chair, engaged in questioning which went significantly outside the scope of the invitation, the court said.
The Dáil Committee on Procedure said it noted today's judgement at its meeting this morning and would not be commenting further until it has considered the ruling in full.