Mahoney, who plays Marty Crane, father of the manic psychiatrist and his brother Niels in the sitcom, officially opened the 25th festival in the Galway Bay hotel.
The TV favourite, who is an avid fan of the stage, is also starring in a play, The Drawer Boy, which is tipped to become one of this year's festival highlights.
This Friday he will, under the direction of Lynne Parker, take to the stage for the play's Irish premier at Galway's Town Hall Theatre.
But, as always, the 14-day festival extravaganza promises much more with theatre and dance companies from all over the world scheduled to perform.
The festival's Big Top dominates the skyline and the banks of the Corrib.
The tent is one of the largest ever erected in Ireland at more than 65 meters long and 40 meters wide. It had to be big as last night it played host to an equally big Macnas world premier.
Macnas' latest project, Gráinne Mhaol, involves over 300 people.
Next week Brazilian dance company Deborah Colker will perform a theatrical spectacular which has already enjoyed considerable success worldwide.
The show promises to be a riveting performance, but Mr Mahoney's presence in Galway is bound to be a crowd-puller as well.