The protected disclosure made by former garda press officer Supt David Taylor "is essentially a work of fiction", lawyers for An Garda Síochana have told the Charleton Tribunal.
Shane Murphy SC was speaking during his closing submission to the tribunal, which is examining allegations from Supt Taylor that he was directed to brief negatively against garda whistleblower Sgt Maurice McCabe.
Former commissioner Martin Callinan denies giving any such direction, and his then deputy Nóirín O'Sullivan denies she was aware of such a direction.
"There is no substantiated evidence that Mr Callinan or Ms O'Sullivan instructed or directed any such effort on Supt Taylor's part to malign Sgt McCabe, or that either were aware of any such effort," Mr Murphy told Mr Justice Peter Charlton, the tribunal chairman.
Mr Murphy said that Supt Taylor's 2016 protected disclosure was "essentially a work of fiction", and the chairman should find that the allegations that Supt Taylor was directed to negatively brief against Sgt McCabe were untrue.
"We say that such a finding, if made, will serve the public interest in establishing how a mere allegation can become the received truth at enormous cost to the individuals involved and at enormous cost to our society," Mr Murphy said.
"It will also serve the public interest in at least partially addressing the substantial harm to the reputation and professional standing of former commissioner Callinan and former commissioner O'Sullivan," Mr Murphy said.
Mr Murphy said that the tribunal had shown the ease with which allegations of misconduct could gain public traction, but a different picture emerged when they were tested in evidence under oath.
"What has emerged from the evidence is a very different picture to what was contained in the allegations in the protected disclosure," Mr Murphy said.
Mr Murphy said that Supt Taylor's credibility was "effectively shattered".
Counsel said that the existence of a rumour in 2014 did not of itself ground an inference that it must have been spread in a deliberate way by Supt Taylor, and some journalists had said they heard rumours as early as 2011, before Supt Taylor became press officer.
Mr Murphy said that Supt Taylor was upset, angry and bitter at being moved from his role in the garda press office.
Submissions continue tomorrow with Mr Murphy, followed by barristers for Independent News & Media and Sgt McCabe.