The letter was sent under Section 41 of the Garda Síochána Act, which requires the commissioner to keep the justice minister fully informed of various issues.
Last week, the head of Garda human resources, John Barrett, told the Public Accounts Committee he had raised formal written concerns about the tribunal department to the Garda legal affairs unit. He is believed to be concerned the tribunal unit is not reflecting the interests of all gardaí.
At the sitting of the PAC, Mr Ó Cualáin refused to answer any more questions about the garda tribunal section as his answers could interfere or undermine the tribunal.
Last night, the Department of Justice confirmed the minister received a section 41 letter this week, it had been referred to the tribunal, and the matter was also receiving careful consideration within the department.