The Supreme Court has found that the Oireachtas Public Accounts Committee acted unlawfully in its questioning of former Rehab CEO Angela Kerins.
Ms Kerins had taken legal action against the committee - claiming questions she was asked as part of a 2014 investigation into money paid to the group amounted to a "witch hunt" against her.
She said the grilling over her €240,000 salary and bonus payments led to her resignation and damaged her career.
She said she answered the questions even though neither issue was listed in the terms of reference for the hearing - and said she suffered from shock, stress and anxiety after the appearance.
In January 2017, the High Court ruled against Ms Kerins - finding that the courts could not intervene in Dáil committee proceedings, as anything said during the hearings is protected by parliamentary privilege.
In that ruling, Mr Justice Peter Kelly said the action attempted to make the Oireachtas amenable to the jurisdiction of the court - which could impair the effective functioning of Parliament.
He warned that this was expressly forbidden in the Constitution.
Ms Kerins then took the challenge to Supreme Court.
This morning, the court ruled that the PAC acted "significantly" outside its powers during the hearings.
The decision is viewed as highly significant in legal and political circles and submissions will be heard in April as to what legal declaration should be made by the court.