Fianna Fáil leader Micheál Martin and former Labour leader Pat Rabbitte will both appear as witnesses at the Charleton Tribunal on Monday.
The tribunal is investigating allegations of a Garda whispering campaign against whistleblower Sgt Maurice McCabe.
Mr Martin met Sgt McCabe four years ago and examined a dossier he had prepared about alleged malpractice and wrongdoing within the Garda Síochána.
It was on foot of Mr Martin’s speech in the Dáil that then taoiseach Enda Kenny agreed to a process that culminated a year later in the O’Higgins Commission.
Mr Martin will be asked if he had been contacted or informed of any false allegations being made about Mr McCabe’s character by members of the force.
Mr Rabbitte, a former TD for Dublin South West, is expected to tell the tribunal how had been contacted by a garda after a radio interview in which he described Mr McCabe as a “credible person”.
He is expected to say the garda who contacted him alluded to false allegations of sexual abuse on the part of Sgt McCabe.
