Former Garda press officer Dave Taylor had 11,000 contacts with journalists during a four-month period after he left the role, the Disclosures Tribunal has heard.
The extent of the contact was discovered by gardaí investigating whether information about the taking into care of a Roma child in October 2013 had been unlawfully disclosed to a reporter.
Supt Taylor was identified as a person of interest in the inquiry after phone records appeared to contradict his claim that he first learned of the matter from the press. The inquiry was later broadened out to include 12 distinctive disclosures of sensitive information to journalists.
Chief Supt Frank Clerkin, who led the investigation, said he sent a file to the Director of Public Prosecutions in August 2015 recommending that Supt Taylor be prosecuted for unlawful disclosure of information.
However, an official at the Office of the DPP directed no prosecution, saying: "While highly suspicious, I do not think that it is possible to prove beyond reasonable doubt that the suspect disclosed confidential information."
Supt Taylor, who was suspended at the time, was then reinstated to the force.
Chief Supt Clerkin gave evidence of checks the inquiry team did on communications involving Supt Taylor between 2013 and 2015. Supt Taylor was transferred out of the Garda Press Office to the traffic bureau in June 2014, but the investigation found he maintained contact with certain journalists.
Michéal P O'Higgins SC, counsel for a number of senior gardaí, said Supt Taylor appeared to have been running "a parallel press office".
"He certainly provided a lot of information to particular journalists, yes," agreed Chief Supt Clerkin.
The tribunal heard that from September to December 2014, Supt Taylor had over 11,000 contacts with journalists. Some 2,858 of these were with Eavan Murray, crime correspondent with 'The Sun'. There was also a much smaller number of contacts with 'Sunday Times' journalist John Mooney and Juno McEnroe of the 'Irish Examiner'. Supt Taylor has claimed he briefed all three negatively about Garda whistleblower Sgt Maurice McCabe.
The tribunal is investigating claims by Supt Taylor that he was directed by former commissioner Martin Callinan, and with the knowledge of then deputy commissioner Nóirín O'Sullivan, to spread a false sexual assault allegation against Sgt McCabe.
Chief Supt Clerkin said his investigation suggested Supt Taylor passed press clippings and critical incident reports from the Garda press office to journalists.
Some 26 stories, labelled "exclusive", were published by Ms Murray which he believed were drawn from critical incident reports.
These included stories about the burglary of the home of a minister, a tiger kidnapping, and two rapes.
Chief Supt Clerkin said he took the issue "extremely seriously" as he felt Garda security systems were being compromised. "I felt it was outrageous what he was doing," he said.
Earlier, a civilian member of the Garda Press Office, Chrissie Fitzpatrick, said Supt Taylor "felt he had been shafted" when he was transferred and blamed Ms O'Sullivan and Garda director of communications Andrew McLindon.
Civilian press officer Brenda O'Grady also said Supt Taylor was annoyed with his transfer and saw it as "a demotion" and "an insult". The tribunal heard he had "a rant" on the phone with her and "was giving out" about Ms O'Sullivan.
Supt Taylor also rang her asking for a list with the contact details of 600 journalists.
Although she printed off the list, she had second thoughts about giving it to him as she felt it was an inappropriate request.