The abuse claims were made by a young woman in August to a counsellor, who contacted Tusla and gardai. However, no attempt was made to contact Mr McCabe and put the allegations to him.

In 2014, Tusla admitted a mistake had been made and attributed the false accusation to a “clerical error”.

It was only last year that Mr McCabe became aware that the highly damaging false abuse allegation had been widely circulated. He is to take a legal case against Tusla and has met with Minister for Children Katherine Zappone. She has indicated a public apology will be forthcoming.

However, major questions remain as to how the allegation came about, how it was processed by both the gardai and Tusla, and why Sergeant McCabe was never informed about it.

The allegation surfaced on a file in August 2013, and the “error” was detected the following May, a period during which Sergeant McCabe’s claims of malpractice were causing major political and garda related controversy.

The allegation was known among senior officers in the force. Despite this knowledge Sergeant McCabe was not informed about it, either after the initial file was created, or once it was discovered to be an error. He was not arrested or questioned about the initial allegation, and neither was he informed by the commissioner, his employer, about the error after May 2014.

Today’s revelation puts in context the rumours and propaganda that has been swirling around Sergeant McCabe since he brought forward his claims of malpractice in the force.

Yesterday, Labour leader Brendan Howlin told the Dail that he had been contacted by a journalist who told him he had direct knowledge of the garda commissioner, Noirin O’Sullivan, briefing journalists that Sergeant McCabe was responsible for “sexual crimes”.

Mrs O'Sullivan has denied spreading the allegations of sex crimes against Mr McCabe.

In a statement yesterday, she said she was surprised by claims of her involvement in a smear campaign targeting Mr McCabe and insisted it was the first time she had heard the accusation.

The explosive allegations about a sex crime slur were revealed by Labour leader Brendan Howlin who said he was contacted by a journalist who claimed to have direct knowledge of the Commissioner being in contact with other reporters.

Ms O'Sullivan said she was taking an unprecedented step to publicly deny the claims despite a judge-led inquiry being ordered.

A statement from her office on Wednesday said: "The Commissioner has no knowledge of the matters referred to by Deputy Howlin and refutes in the strongest terms the suggestion that she has engaged in the conduct alleged against a serving member of An Garda Siochana.

"This is the first occasion on which the Commissioner has been made aware of the allegations made by Deputy Howlin and to her knowledge no report having been made to the Garda Siochana Ombudsman or elsewhere relating to the specific allegations."

Judge Peter Charleton has been appointed to lead an inquiry into allegations that senior officers attempted to blacken Sgt McCabe's name among the media with unfounded allegations.

He will examine nine issues including whether Commissioner O'Sullivan knew about it or if she played any part in directing it.

The Commissioner said Judge Charleton will get full cooperation from the force.

Listen to Michael Clifford's interview with Matt Cooper on this story via the link below. His interview starts 9mins 22 sec in.