The former Garda Commissioner Noirin O'Sullivan is due to appear before the Disclosures Tribunal today.
She had been expected to attend on Friday, but her testimony was delayed.
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's 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.