The inquiry will investigate the file of false allegations of sexual abuse against Sgt Maurice McCabe
A Garda colleague of Sgt Maurice McCabe alleged the whistleblower sexually abused his young daughter.
The Disclosures Tribunal has been hearing evidence from the social worker involved in the case this morning.
Social worker Rhona Murphy had been dealing with a young girl known as Miss D, who was showing difficult behaviour since 2005.
Miss D made an allegation of sexual abuse which occurred in 1998, when she was 6 or 7.
The retrospective allegation was that during a game a man held her over a sofa, they were fully clothed but there was humping involved.
Her father - who was a guard - claimed Maurice McCabe was the man involved.
The DPP decided not to prosecute with social workers involved deciding it was inconclusive and closing the case in 2007.
They said there were no complaints from other parties against Mr McCabe.
The first day of evidence in the Disclosures Tribunal continues this afternoon at Dublin Castle, with several other witnesses scheduled to appear.
The tribunal will investigate whether the false sexual abuse allegations "were knowingly used by senior members of An Garda Siochana to discredit Sergeant McCabe".
The treatment of other Garda whistleblowers will be looked at later in the inquiry.
Although a smaller-scale Commission of Investigation was originally planned, earlier this year it was changed to a full public inquiry amid public and political outrage over the allegations.
Supreme Court judge Peter Charleton is leading the public inquiry.