Some 20,000 people have stopped tuning in to Today FM's afternoon slot since Al Porter quit his role as presenter.
The 25-year-old decided to leave after allegations of inappropriate behaviour were made against him.
According to the latest Join National Listenership Research figures, the number of people listening to that slot has dropped from 118,000 to 98,000.
Last month, Muireann O'Connell was announced as Porter's permanent replacement on the show.
A statement from Today FM said they expected figures to return to their former heights eventually.
"Naturally when there is a schedule change, either planned or unexpected, there will always be a knock-on effect on the core audience who would have been loyal to that show," it read.
"Today FM had an unexpected change to its schedule in early November, and consequently, this has had an effect on listening audiences.
"We are confident that the addition of our new and extremely popular presenter Muireann O Connell will reverse that change."