A garda who identified himself by using the Nike logo supplied the barrister representing the McBrearty family with information during their fight to clear their name.
He may also be the source of anonymous allegations against senior officers which led to the setting up of the Morris tribunal.
Tribunal barrister Anthony Barr SC said that barrister Martin Giblin SC had told the inquiry he was first contacted by the source in the autumn of 1997.
Mr Giblin said that the anonymous source, who identified himself as "a garda based in Donegal", sent him documents in envelopes marked with the Nike sports symbol.
He said that when he received a fax from his client Frank McBrearty snr on June 25th 2000, alleging that the Carty inquiry was compromised, he "was intrigued by the contents" and spoke to Mr McBrearty snr, who told him that the document had been received anonymously.
"I felt I recognised the author of the document as my anonymous informant, from whom I have not heard for a considerable period," Mr Giblin said.
As a result of receiving the document, Mr Giblin contacted Brendan Howlin, the Labour Party's spokesman on justice. Mr McBrearty snr faxed the document to Fine Gael's justice spokesman, Jim Higgins (now an MEP), on the same day. Both TDs met the then minister for justice, John O'Donoghue, some days later.
Mr Higgins received a second fax from Mr McBrearty snr the following month.
The tribunal heard that no evidence had been found "to support any of the allegations advanced in either of the faxed documents".
In a statement, Mr Higgins said Mr McBrearty snr told him the statement came from "my friend", a phrase Mr McBrearty snr used to refer to P.J. Togher, a retired detective garda.
In direct evidence, Mr Togher said that he was not the author of the allegations and had not even seen the document until it was shown to him by tribunal investigators. The retired detective garda also said that a disputed statement of admission allegedly made by Frank McBrearty jnr "was obtained by a trick", and this was the impression he wanted to convey to Mr Higgins during a meeting in 1999.
Mr McBrearty jnr has denied making any admission in Garda custody.
Mr Togher said that Mr McBrearty snr did not discuss the anonymous allegations document with him before forwarding it to Mr Higgins, and he did not know "why he sent it or what possessed him to send it".