The Lovely Bones star has been digitally edited into scenes from movies including My Left Foot and Once, and documentary footage of John F. Kennedy's arrival at Dublin Airport and an Irish football match as part of an awareness boost for the Irish Film Institute's Irish Film Archive.
Officials are hoping the ad campaign will help them raise $395,000 (£247,000) to build a new centre to house cans of film.
In the clip, Ronan says, "The written word can be powerful and beautiful but films transport us to another place in a way that even the most evocative words never can.
"If we lose our films we lose that other place forever. The IFI Irish Film Archive urgently needs your support to save and preserve our film heritage."
The IFI currently stores 27,000 movie cans and 10,000 broadcast tapes preserved from every decade since the first Irish cinema opened in 1909.