According to The Hollywood Reporter, the two actresses have been cast as the leads in The Crying Game director Neil Jordan's next project, Byzantium.
The film, from an original screenplay by Moira Buffini, follows a mother who turns her own daughter into a vampire, with the two forming a deadly partnership together.
"I love the horror genre and we've been working on this for three or four years," producer Stephen Woolley said of the project.
It was recently confirmed that Ronan would star as the lead in an adaptation of Twilight author Stephanie Meyer's supernatural novel The Host.
Byzantium will begin filming this October.
Watch Saoirse Ronan in the trailer for Hanna below: