RTÉ paid Ryan Tubridy €345,000 more than declared from 2017 to 2022.
The broadcaster has described it as a "serious breach of trust".
A review was carried out by Grant Thornton into RTÉ's accounts - which was discussed by the board of the broadcaster on Monday.
It found payments totaling €345,000 were paid to Ryan Tubridy between 2017 and 2022 - over what was made public.
The review found RTE had guaranteed additional payments to the former Late Late Show host through commercial partners - in exchange for appearances.
Ryan Tubridy stepped down as host of the Late Late Show earlier this year.
Media Minister Catherine Martin says she is extremely concerned with the findings and has set up a meeting with the broadcaster's chair on Monday.
The RTÉ Trade Union Group has expressed shock and disappointment at today’s statement from the chair of the RTÉ Board in relation to undisclosed payments made to Ryan Tubridy.
Speaking after a meeting of the Officers and Officials of the group of unions, Chair of the TUG, Stuart Masterson, said RTÉ has done immense damage to the relationship with staff.
He added it happened at a time when staff were engaged in cost-cutting negotiations with management.
