The former Garda Commissioner Nóirín O’Sullivan is due to appear before the Disclosures Tribunal today.
She had been expected to attend on Friday, but her testimony was delayed. Her appearance is expected to begin this afternoon.
The Disclosures Tribunal was set up to look at the treatment of Garda whistleblower Maurice Mc Cabe.
Ms O’Sullivan will be asked about the legal strategy used to discredit him after he exposed wrong doing in the force.
Mr Justice Peter Charleton, who is heading up the Tribunal, says Commissioner O’Sullivan did not rely on false sex abuse allegations, but says questions remain about whether she used other inappropriate grounds to undermine Sergeant McCabe.