Former detective sergeant Hugh Smith said the arrest and detention of electrician Damien McDaid in December 1996 was uneventful.
Mr Smith said that he believed there was little evidence to arrest the father-of-two at the time and therefore gardaí had no reason to interrogate him with force.
Mr McDaid, 41, from Newtowncunningham, was held in Letterkenny Garda Station by gardaí investigating the death of cattle dealer Richie Barron, a hit-and-run victim who they wrongly believed had been murdered.
He claims retired detective garda Sean Herraghty ordered him to his knees and pointed a handgun in his mouth, spat on his breakfast and jumped up and down on his feet.
Mr McDaid says Detective Garda Martin Anderson pushed him in a corridor just moments before his release, while retired Sergeant Martin Moylan is said to have shouted and roared at him during questioning.
Gardaí deny all the allegations.
“I am mystified by the allegations claimed to the tribunal team,” said Mr Smith, who later became Mr Herraghty’s sergeant.
“The allegations are the most serious allegations I have ever heard against a member of An Garda Síochána any place in the country.
“The Sean Herraghty being described here is not the Sean Herraghty I know. I wouldn’t tolerate that behaviour.
“And in circumstances it did happen Sean Herraghty should be languishing in jail and not giving evidence to the tribunal.
“It’s a horrible act to even contemplate, let alone to carry out.”
Mr McDaid, who cannot read or write, was arrested as an accessory after the fact of murder.
Gardaí accused him of seeing suspect Frank McBrearty Jnr with an iron bar on the night Mr Barron died. He was also suspected of hiding an iron bar for Mr McBrearty.
Mr McDaid maintained he had no idea where he had been on that night as he had been out drinking all day.
The electrician claimed that gardaí had written a statement which claimed he was with Mr McBrearty Jnr on the night of Mr Barron’s death and when he refused to sign the statement the interview turned nasty.
The Morris Tribunal is investigating claims that some 12 people — many related to the McBrearty family — were interrogated, intimidated or abused during the botched investigation.