Nóirín O'Sullivan has said she was concerned that the resignation of a whistleblower, who said he felt under threat from the former commissioner, would create a "public and political storm".
Sergeant Maurice McCabe resigned from his position as head of the traffic unit in Mullingar, Co Westmeath, shortly after learning that Garda lawyers intended to attack his integrity at a private inquiry into his claims of corruption and wrongdoing within the force.
He said he felt "under threat" from the then commissioner and was worried that if he made any mistakes management would "come down on me like a tonne of bricks".
Giving evidence to the Disclosures Tribunal for a third day, Mrs O'Sullivan said she had no idea why Sgt McCabe felt under threat from her and was concerned by the potential backlash his resignation could cause.
"My concern was, the fact that the prominence Mr McCabe had, if this matter was to become public, the public and political storm that would create.
"I was at a loss to understand how he felt under threat from me. I was not able to establish in my own mind why it was he was behaving that way," said Mrs O'Sullivan.
The retired police chief added: "It is very difficult to try and figure out what is going on in somebody else's head."
She insisted that she had ensured proper support was available for Sgt McCabe in the workplace.
Mrs O'Sullivan also told the tribunal that she was not aware of any potential conflict of interest arising out of the fact she was represented at the O'Higgins Commission by the same lawyer representing a superintendent who believed Sgt McCabe was out to ruin his career.
The tribunal heard that Superintendent Noel Cunningham told the O'Higgins Commission that he believed Sgt McCabe was out to undermine his career.
Sgt McCabe's lawyer Michael McDowell said: "That suggested very strongly that Superintendent Cunningham was asking the commission to draw a poor view of Sergeant McCabe as a man who had undermined his career.
"You said you wanted to be impartial. Superintendent Cunningham was vehemently of the view that Sgt McCabe was trying to undermine him.
"There was a conflict of interest if you were trying to be an impartial referee. You were using as your mouthpiece a counsel representing a man who was trying to say Sgt McCabe was trying to ruin his career," said Mr McDowell.
Mrs O'Sullivan responded: "At no stage was I told a conflict of interest had arisen and it was necessary to review arrangements."
The Disclosures Tribunal at Dublin Castle is investigating whether unjustified grounds were used in an attempt to discredit Sgt McCabe at the private O'Higgins Commission in 2015.
That inquiry examined the sergeant's claims of wrongdoing among gardai in Cavan-Monaghan.
Part of the Disclosures Tribunal's terms of reference is considering whether a false and unfounded child sex abuse allegation made against Sgt McCabe in 2006 was used against him.
It is also investigating whether Mrs O'Sullivan's lawyers advised that Sgt McCabe's motivation should be challenged at the private inquiry despite public support for whistleblowers.
