Businessman Denis O'Brien's appeal against the dismissal of his action against the Dáil and State, over statements made by two TDs in the Dáil about his banking affairs, will open before a seven-judge Supreme Court on Monday.
Mr O'Brien is also appealing an order requiring him to pay the estimated €1m legal costs of his failed High Court action.
His appeal, listed for two days, follows a separate Supreme Court appeal by former Rehab CEO Angela Kerins against the rejection of her challenge over her treatment at two Dáil Public Accounts Committee hearings into payments to Rehab.
Both appeals raise similar issues concerning the constitutional separation of powers and the courts' ability to intervene in decisions and proceedings of Houses and Committees of the Oireachtas.
Ms Kerins's appeal was heard over four days last month and judgment was reserved by the seven judge court comprising the Chief Justice, Mr Justice Frank Clarke, Mr Justice Donal O'Donnell, Mr Justice William McKechnie, Mr Justice John McMenamin, Ms Justice Elizabeth Dunne, Ms Justice Iseult O'Malley and Ms Justice Mary Finlay Geoghegan.
The Public Accounts Committee’s appeal against the High Court’s decision directing it to pay Ms Kerins’ estimated €700,000 costs of the case there, plus the State’s appeal over an order requiring it to pay its own costs, will be dealt with after the Supreme Court has given judgment.
The same seven judges will hear Mr O’Brien’s appeal, brought against the clerk of the Dáil, the Dáil Committee on Procedures and Privileges and the State.
His proceedings arose from statements made on separate dates in summer 2015 by Social Democrats TD Catherine Murphy and Sinn Féin TD Pearse Doherty after Mr O'Brien got court injunctions restraining RTÉ publicising that information.
In her March 2017 High Court judgment dismissing the case, Ms Justice Una Ní Raifeartaigh held that what Mr O’Brien sought was prohibited by the separation of powers under the Constitution and would have a “chilling effect” on parliamentary speech into the future.
She upheld the core defence argument that Article 15 of the Constitution immunises Dáil “utterances” from suit or scrutiny by courts or tribunals and later awarded the estimated €1m costs of the action against Mr O’Brien.