He also claims that Callinan’s deputy, Nóirín O’Sullivan, knew of this. Since making those original allegations he has added that the director of communications in the force, Andrew McLindon, was also aware of the smear campaign.

On the other side, he has named 11 reporters whom he claims to have briefed negatively about McCabe. All of them are either denying this outright or claiming journalistic privilege, although Taylor has waived his privilege as a journalistic source.

Apart from effectively incriminating himself in this alleged campaign, Taylor had provided next to nothing by way of corroboration.

Over three days of testimony he was asked again and again for specifics, or even ballpark estimates, of how and where he did this briefing and how he kept O’Sullivan informed. Each time he fielded such questions, he resorted to generalities. He has no recollection of specifics.

He was transferred out of the press office when O’Sullivan succeeded Callinan in March 2014. Former colleagues have said that he was “bitter” at the move.

Yesterday, it was put to Taylor that he had sent a text some months later, when he was based in the traffic division in Dublin Castle, saying, “I’m currently in the dungeon in Dublin Castle awaiting parole.”

He was asked whether he had told a colleague that he was “gutted” when O’Sullivan was appointed as commissioner.

She wouldn’t be my number one choice, but that’s just me,” he replied.

When he made his protected disclosure in September 2016, he was under suspension and the threat of prosecution. Counsel for the former commissioners, Micheal O’Higgins, brought Taylor through the detail of his life in leading up to the protected disclosure. The witness claimed that his thinking “evolved” while he was suspended, to the point where he concluded that it had been wrong to smear McCabe.

O’Higgins has a different theory. He put it to Taylor that he had been “fixated with Nóirín O’Sullivan and her husband’s role in this investigation [into the leaks] and you were lashing out and trying to paint yourself not as a wrongdoer but drawing parallels with Sgt McCabe”.

The lawyer went on to suggest that nobody would have believed an allegation about a smear campaign run by O’Sullivan, so “you had to include him [Callinan[ in the whole story”.

I reject that,” replied Taylor.

The tribunal has heard evidence from a number of witnesses that Callinan briefed them negatively about McCabe.

Two of these witnesses — TD John McGuinness and Comptroller and Auditor General Seamus McCarthy — had stated that Callinan described McCabe as a child abuser. Callinan denies this.

The tribunal has also been provided with a statement from RTÉ journalist Philip Boucher-Hayes that, in December 2013, Callinan spoke in disparaging terms about McCabe, and told him that if he wanted more detail he should talk to Taylor. The scenario would suggest that Callinan had enlisted his press officer to brief negatively about McCabe, if Boucher-Hayes’s evidence were accepted.

Beyond that, there are only phone records of contact between Taylor and journalists on one side, and Callinan and O’Sullivan in Garda HQ.

In his role as press officer, Taylor would have been in regular contact with journalists so the records themselves have little probative value. The tribunal has heard that 12 of 15 phones that were used by Taylor and the two former commissioners are no longer available for examination.

So evidence of a smear campaign as alleged by Taylor is thin on the ground. There is no doubt McCabe, who was highlighting abuse of the penalty points system, was certainly unpopular in Garda HQ.

However, the allegation that there was an organised smear campaign is the matter at issue for the tribunal.

Taylor’s allegation has also introduced major issues for the media.

He has waived journalistic privilege, but a number of those he has named are claiming privilege nonetheless.

Tribunal chairman Peter Charleton has given indications that he is taking a dim view of this. There remains the threat of a diversion down to the High Court to resolve any issue over journalistic privilege.

Former commissioner Martin Callinan’s evidence is expected to begin today.