The final outcome of her case awaits subsequent court findings on the extent of the Public Accounts Committee’s unlawful conduct. After that, we will need to wait to see what the judgment will mean for the workings of Oireachtas committees.

Before the Kerins judgment — the Supreme Court ruled that the PAC acted outside its remit in its questioning of the Rehab former chief executive — the prevailing view was that the courts did not have any substantial oversight of what is said in Oireachtas committee hearings. This is because the Constitution protects — in the strongest terms — speech in the Oireachtas.

Article 15.13 states that the members of the Oireachtas 'shall not, in respect of any utterance in either House, be amenable to any court of any authority other than the House itself'

This protection for parliamentary speech is, as the Supreme Court noted, a crucial part of democracy. It lets the Oireachtas govern its own procedures to ensure a robust and free exchange of views on matters of national importance and without the fear of litigation.

Members are not unaccountable: they can be held to account by their peers in the Oireachtas, for any abuse of their privileges, as well as by the electorate, but they are not answerable in court.

This is a core part of the constitutional separation of powers, limiting judicial involvement in the Oireachtas, allowing the legislature to work freely and effectively. In the Kerins case, the Supreme Court has nuanced this position.

An Oireachtas committee, the court held, usually enjoys the same robust protection for speech as the Dáil or the Seanad, as they undertake crucial parliamentary work in respect of legislation and policy-making. But if a committee goes too far — if it goes beyond its brief; exceeds its terms of reference — it loses some of this protection, and may be answerable in the courts.

In the Kerins case, the PAC had clearly acted beyond its powers. It had no power to investigate Rehab and its CEO, Angela Kerins, as the body — though in receipt of some public funds — was not subject to the jurisdiction of PAC. The Committee on Privilege and Procedure (CPP) — which oversees and enforces the rules of the Oireachtas — later found that PAC had exceeded

its powers and gone beyond its mandate.

The High Court noted that the incident had damaged Ms Kerins’ reputation. The Supreme Court said that since the protection that the committee enjoys comes from the fact that it is delegated power to perform certain tasks by the Houses of the Oireachtas, if it leaves those tasks behind and starts to do something else, it may be answerable for any illegality or breach of rights that result.

It highlighted that PAC crossed a line in this instance and that PAC had departed substantially from the topics it had outlined in its invitation to Kerins.

Moreover, the court found, the CPP had not sanctioned PAC in any real way for acting outside of its powers.

In these circumstances, the court held, the judiciary was entitled to consider the actions of the committee, to conclude if any unlawful actions had been taken, and to give remedies when appropriate. At a subsequent hearing, the Supreme Court will consider the detail of PAC’s conduct to decide whether, in this instance, it crossed the threshold for judicial intervention.

Throughout the judgment, the court was very clear that Oireachtas committees deserved substantial leeway in conducting business. The courts should not intervene for minor errors, or technical matters, or matters that relate to the core of the legislature’s functions, only “substantial and significant” deviation from proper conduct.

The court was keen to stress it does not want to unduly obstruct the work of the legislature, only to balance this with the rights of individuals when the work of committees exceeds the scope of their mandate.

But striking this balance is easier said than done. How will we establish where the lines are drawn, where the committee steps too far over its boundary, where it strays too far from the terms of the invitation?

The Supreme Court said it will decide this case by case, having regard to “all the circumstances of the case, while affording a very significant margin of appreciation to the Houses as to [how] they conduct their business”.

The risk is that this will not be sufficiently clear in practice, at least not until the court has had more opportunities to consider such cases and establish guidelines. If it is uncertain, Oireachtas committees may have to seek regular legal advice about terms of reference and the appropriateness of particular lines of questions.

This legal advice may take a ‘better safe than sorry’ approach and advise committees to be very cautious in investigation and questioning. This could create a chilling effect, where fear of litigation and court oversight hampers the freedom of parliamentary speech, the very thing that Article 15.13 aims to avoid.

We have seen this before, in the aftermath of the Abbeylara case, where the Supreme Court placed strict limits on the ability of Oireachtas inquiries to make findings against individuals. The stricture of the court’s judgment, combined with fear of possible litigation, has led to many problems in the conduct of public inquiries, notably during the banking inquiry.

It is too early to say what effect the Supreme Court judgment will have on the behaviour of Oireachtas committees. Perhaps committees will still have robust debate, while better protecting individual rights.

But perhaps, in practice, committees will do a lot less, out of fear of possible courts cases.

The Supreme Court is trying to balance protecting the legislative power with protecting the rights of citizens, but, in doing so, the court has chipped away at the line, drawn in the constitutional separation of powers, between the legislature and the courts.

As to whether or not this will have potentially serious effects on the workings of the legislature, only time will tell.

Dr David Kenny is assistant professor of law at Trinity College Dublin. He is co-author of the recent fifth edition of the leading text on Irish constitutional law, Kelly: the Irish Constitution.