Angela Kerins has said she is "very pleased" after the Supreme Court ruled that the Public Accounts Committee (PAC) acted unlawfully in its questioning of her.
The six judge court made the ruling on Wednesday morning after determining in February that it had to power to do so.
It said "by conducting a public hearing in a manner which was significantly outside of its terms of reference and which also departed significantly from the terms of an invitation by virtue of which a citizen was requested to attend, the Public Accounts Committee (PAC) of Dáil Éireann acted unlawfully."
Ms Kerins, the former Rehab CEO, went to the Supreme Court after the High Court rejected her challenge over how she was treated at two PAC hearings in 2014.
The hearings were over payments to Rehab, where she was asked about her €240,000 annual salary.
Ms Kerins claimed the hearings amounted to a witch-hunt - and she sought damages from the committee for public humiliation and the loss of her career.
In February, the Supreme Court ruled that it had the power to declare that PAC acted unlawfully in its treatment of Ms Kerins.
On Wednesday morning it formally made that declaration.
It said that the hearing was conducted outside of its terms of reference and departed from the terms on which Ms Kerins was invited to attend.
The court made the ruling against the Dáil rather than individual members of PAC.
The judgment has set a precedent and could influence the way Oireachtas Committees are conducted going forward.
"An enormous decision for me"
In a statement released through her solicitors, Ms Kerins 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.
"I would like to thank those closest to me, my family, my friends and my exceptional legal team for their unrelenting support and guidance.
"It was an enormous decision for me to take this case and I could not have done it without them.
"Over the past five years I have been deeply moved by the kindness; compassion and support shown to me by the many people who contacted me by letter; email and by phone and sometimes in the street.
"I thank them from the bottom of my heart."
Reporting by Kim Buckley and Jack Quann