In a significant judgment, the Supreme Court has found that the Dáil's Public Accounts Committee acted significantly outside of its terms of reference in its dealings with the former Chief Executive of the Rehab group, Angela Kerins.
The court found it has the power to declare the PAC's actions unlawful in the light of the fact that the committee was acting outside its remit and that a relevant Dáil committee had come to the same view.
The High Court found the courts had no such power.
The Supreme Court said it had concluded the Dáil itself was the appropriate defendant in proceedings concerning the conduct of a Dáil committee and it was not appropriate to name the individual members of a committee as defendants.
This is one of the issues that has to be resolved before the court will make a declaration that the PAC acted unlawfully.
It also wants submissions from the parties on the issue of whether or not the committee, as a whole, breached the terms of the invitation it extended to Ms Kerins to appear before it.
There will be a further hearing on 8 April.
A spokesperson for the Houses of the Oireachtas has said today's judgment will be considered.
The spokesperson said that the Oireachtas would not be commenting further at this point.
The core issue in the case was Ms Kerins'contention that the PAC acted outside its powers in its dealings with her, and whether someone in her position had a remedy before the courts.
In the ruling delivered by Chief Justice Mr Justice Frank Clarke, he said a court could consider evidence of what was said at an Oireachtas committee meeting for the purposes of determining what actions the committee was engaged in.
He said this did not breach Article 15 of the Constitution which says members of the Oireachtas cannot be "amenable" to a court in relation to any of their utterances.
He said the court found that the privileges and immunities of the Oireachtas were extensive but did not provide an absolute barrier to bringing proceedings concerning the actions of an Oireachtas committee.
However the Court said it recognised that proceedings could not be properly brought which would involve the Court breaching Article 15 or would amount to an inappropriate breach of the separation of powers.
It found the primary role of providing a remedy where a citizen is affected by unlawful action during the course of the conduct of the business of the Oireachtas lies with the Houses themselves and they have a very wide "margin of appreciation" as to how their business is conducted.
Mr Justice Clarke said the courts could only intervene where as a result of an assessment of all the circumstances of the case, there had been a significant and unremedied unlawful action by a committee.
What requires to be assessed, he said was the actions of a committee as a whole, not individual utterances of committee members.
In this case he ruled,it would not be a breach of the separation of powers to declare the actions of the PAC unlawful, in light of the fact that it was acting outside its terms of reference and a relevant Oireachtas committee - the Dáil Committee on Procedure and Privilege had come to the same conclusion.
He said it might be appropriate to find on the evidence that the committee had invited Ms Kerins to attend before it on one basis, but when she attended, proceeded to deal with her on an entirely different basis.
Ms Kerins was written to in early 2014 advising her that the PAC was currently examining the issue of State funding to the Rehab group.
In February that year, she was invited to appear before it as it examined payments made to Rehab by state agencies.
At the hearing on 27 February she was questioned on issues including her pension, salary and family members' business dealings.
Following the hearing she said she suffered from shock, stress and anxiety and later attempted to take her own life.
She informed the committee she would not be able to attend a further hearing in April.
Mr Justice Clarke said Court would require further submissions from the parties on whether it was appropriate to characterise the actions of the PAC as a whole as having breached its obligation to treat Ms Kerins fairly.
He said the court had indicated that a citizen who accepted an invitation to attend before a committee of the Oireachtas was entitled to expect the committee would act within the boundaries of the invitation.
He said if the issues concerning this and the question of the proper defendants to the case were resolved in Ms Kerins' favour, the court would make a declaration that the PAC acted unlawfully in respect of Ms Kerins.
The Chief Justice pointed out that damages would by no means necessarily follow from a declaration that the PAC had acted unlawfully.
He said a whole range of issues would need to be considered, including the need to protect the entitlement of freedom of speech in the Oireachtas.
He said it would be necessary to establish a link between the unlawfulness established and any damage suffered.
The issue of damages would have to go back to a further hearing before the High Court he said which would have to address very significant legal issues.