A trawl by the Department of Justice has revealed that the then Garda Commissioner Noirín O'Sullivan personally told a senior official at the Department of Justice about the legal dispute which arose at the O'Higgins Commission, in which her legal team challenged the motivation of garda whistleblower Sgt Maurice McCabe, RTE's This Week programme has learned.

It is the first time that any indication has emerged that the Garda Commissioner personally contacted the Department to discuss aspects of the controversial strategy to challenge Mr McCabe's motives at the Tribunal in May 2015.

The Department of Justice has confirmed to RTÉ that the contact took place.

However, sources say that Tánaiste Frances Fitzgerald was not told of the conversation between Ms O'Sullivan and a senior official at the Dept of Justice.

The Department said the conversation occurred "at the time" of the legal dispute in the Tribunal, but said they had no written record or otherwise and could say no more due to the upcoming Disclosures Tribunal.

It is the second verifiable instance in which the Department, then headed by Ms Fitzgerald, was told about aspects of the controversial legal strategy before it became public the following year.

It later publically emerged in May 2016 that Ms O'Sullivan had instructed her legal team to challenge Mr McCabe's motivation for raising concerns about wrongdoing in the force.

After the legal strategy to attack Mr McCabe's motivation became known, Ms Fitzgerald, who was Minister for Justice at the time, said on the Dáil record in May 2016 that she had no knowledge of the attack on Mr McCabe or of any proceedings which emerged in evidence at the Tribunal.

Among the questions the Department declined to answer was whether the former senior official or anyone else briefed the Minister on the content of the call from the Commissioner, and if not, why not?

The latest revelation has emerged following a detailed series of questions submitted to the Department over several days last week by RTÉ’s This Week programme, who had been told by a security source that a conversation between the former Commissioner had occurred with someone of considerable influence within the Department.

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The source indicated that while a record of the call may not exist in the Department, it may exist in the force.

The Department responded last night to confirm that they had now discovered evidence that the Garda Commissoner did discuss the matter with the Department.

A spokesman said: "In the interests of clarity, we can state that in the course of our wider trawl for records in recent days we contacted a former senior official who stated that he recalled the former Garda Commissioner mentioning to him at the time that a legal dispute had arisen between Senior Counsel at the Tribunal, along the lines of what was set out in the email to the Minister's private secretary on 15 May 2015."

However, they declined to answer a series of related questions, such as whether the Minister was briefed on the call from the Commissioner; was the Disclosures Tribunal told about this contact, and when exactly it occurred.

"The question of contact with the Garda Commissioner on this issue is encompassed by the terms of reference of the Disclosures Tribunal and accordingly the Department is not in a position to comment further in that regard," the Department of Justice spokesman said.