Former Rehab CEO Angela Kerins will take a Supreme Court appeal today in what is described as an "unprecedented challenge" to parliamentary speech.
She has accused the Public Accounts Committee of overstepping its remit when questioning her about her role as a charity boss.
Ms Kerins claims the grilling by PAC amounted to a "witch hunt" and led to her resignation.
The High Court dismissed her case last year, but ordered the State to pay two-thirds of her costs.