The Disclosures Tribunal is to hear today from former Garda Commissioner Martin Callinan.
The inquiry is looking into allegations that Mr Callinan and others in senior management ordered a smear campaign against the whistleblower Maurice McCabe.
The Tribunal previously heard evidence from Fianna Fáil Deputy John McGuinness who said the then Commissioner told him that Maurice McCabe had sexually abused his family.
That is a claim Mr Callinan denies.
The former Minister for Justice Alan Shatter is also due before the tribunal today.