The period drama will be produced by Harry Potter's David Heyman under Heyday Films and BBC Films, reports Deadline.
Testament of Youth tells the story of Vera's experience as a nurse during World War I and the beginning of her career as a journalist and involvement in pacifism.
Ronan, who rose to fame in fellow period drama Antonement, has several projects in the pipeline including Byzantium, The Order of the Seven and Noah.
Next year, the Hanna actress will appear in the film adaptation of Stephenie Meyer's novel The Host as Melanie Stryder, one of the many humans whose body has been inhabited by aliens from another planet.
The Host is scheduled for a March 29, 2013 release in the US and UK.
Watch the trailer for Hanna below: