The Disclosures Tribunal has confirmed that former garda commissioner Nóirín O'Sullivan and former minister for justice Frances Fitzgerald will give evidence during public sessions of the tribunal early in the new year.
Garda whistleblower Sergeant Maurice McCabe is also due to give evidence to the tribunal next month.
The inquiry is investigating whether Ms O'Sullivan tried to discredit Sgt McCabe at the private hearings of the O'Higgins Commission in 2015.
Several senior gardaí along with senior officials from the Department of Justice will also be questioned as the inquiry examines the legal strategy used by the former commissioner at the O'Higgins Commission.
The tribunal returns on 8 January with the garda's head of legal affairs, Ken Ruane, as the first witness.
Ms O'Sullivan is scheduled to give evidence over the following two days.
The tribunal will also hear from her legal team at the O'Higgins Commission - barristers Colm Smyth, Garret Byrne and Michael McNamee.
Ms Fitzgerald and several senior Department of Justice officials will also be questioned about their knowledge of any such campaign.
The former tánaiste and minister for justice resigned last month after emails emerged showing she had been informed of the legal strategy used by the former garda commissioner.
Among those due to give evidence are former department of Justice secretary general, Noel Waters and officials Michael Flahive, Ken O'Leary and Richard Barrett of the Attorney General's office.
Garda head of Human Resources, John Barrett and former chief administrative officer, Cyril Dunne will also be questioned.
Sgt McCabe is penciled in to give evidence on 18 January.