A senior Garda officer told the tribunal he was told there was a "mole" in his office as part of an attempt to undermine his authority.
Supt John Fitzgerald said he received a phone call at home from Chief Supt Denis Fitzpatrick one night after midnight, between 1994 and 1996, telling him, "John, there is a mole in your office, do something about it." The mole was supplying information "in relation to subversives", he said.
Supt Lennon had given him the same information earlier, Supt Fitzgerald said. "I felt it was a farce. I viewed it as speculation."
Supt Fitzgerald said he did not perceive any problem with information leaking in his office at the time.
"I just kept a close eye on my staff and I was quite satisfied that they were sound."
Tribunal chairman Mr Justice Morris asked if Supt Fitzgerald had regarded the information about a possible security leak as "some sort of an undermining process." "That's the way I judged it at the time," he replied.
Supt Fitzgerald served as detective inspector in Letterkenny from January 1990 to March 1992, and as superintendent and district officer until February 1997. Prior to his assignment in Letterkenny he was stationed in Buncrana.
The tribunal is currently examining allegations that Det Noel McMahon and Supt Kevin Lennon prepared explosives for subsequent use in bogus Garda arms finds.
The two officers have both denied those claims.
Supt Fitzgerald was also asked about alleged informer Ms Adrienne McGlinchey, who has denied ever being an informer. Supt Fitzgerald said: "I wouldn't have considered her as important to the gardaí, but having said that people were used by the IRA at the time."
Supt Fitzgerald said that Ms McGlinchey "was a person we would have to keep an eye on".
But he added that "there's no way I would have associated her with being in any active unit of the IRA".