The Lovely Bones star plays a trainee assassin in the thriller and she underwent months of tough training prior to shooting the film.
Ronan admits the active work was hard to begin with, but she slowly adapted to her new lifestyle - and eventually felt the benefit of her new-found exercise program.
She tells Britain's OK! TV, "I started to train a couple of months before we began shooting. I did a couple of weeks of training in L.A. with the stunt co-ordinator and that was really helpful for both of us, especially him as he was able to design a very specific fighting style and I could then go back to Ireland and work on it.
"I was working out in the gym for about two hours every day. For the first few days it was tough, because it's the gym and I had never been in the gym before, but then after that it was fine and I actually enjoyed pushing myself and being pushed. That made me feel a little bit more capable."