Ryan Gosling's directorial debut, which is described as a fantasy-drama, will star Christina Hendricks, Ben Mendelsohn, Doctor Who's Matt Smith and Eva Mendes.
The Gosling-penned story follows a single mother-of-two, who is swept into a dark fantasy underworld after her son discovers a secret path to an underwater town.
How to Catch a Monster will begin shooting this May in Detroit.
Ronan stars in the upcoming adaptation of The Host, author Stephenie Meyer's follow-up to the Twilight saga.
> Stephenie Meyer's 'The Host' debuts new Melanie Stryder poster
> Saoirse Ronan to star in Wes Anderson's 'Grand Budapest Hotel'
Watch Saoirse Ronan discussing The Host with Digital Spy below: