The new drama "How I Live Now" is based on a best-selling young adult novel and focuses on an American teen, played by Ronan, who is sent to live with her relatives in the British countryside.
"She's got a sort of mask on, which hides her vulnerability and her unhappiness," said Kevin McDonald, director of "How I Live Now."
"It's really a coming-of-age story for Daisy. I feel like she's very similar to a lot of young people who do put up that front and are quite difficult to be around," said Ronan.
As if things aren't tough enough just being a teenager in a foreign land, catastrophe strikes -- nuclear war.
Ronan's character, Daisy, has to grow up and fast.
"For me, the film is very much about home and family, and finally she has a home, and she's loved, and that makes it even more heartbreaking when it's all ripped away, so that determination really comes out in her," said Ronan.
McDonald directed a cast that was mostly made up of actors under 20 years old.
"We were all staying in this tiny little town in Wales, a long way from anywhere, staying in little cottages and bed and breakfasts and things around there, so there was nothing else to do but hang out together," said McDonald. "It was a very lovely atmosphere, and it's summer in the countryside, and the sheep are around you."
"There was no sun, but technically it was summer," said Ronan.
"It was the worst summer ever," added McDonald.
Making a film about war didn't faze Ronan but seeing the finished product did.
"To actually sit down and watch it, the only thing that I could really appreciate in an objective way was the fact that this could really happen at any point and at any time; that makes it more hard hitting," said Ronan.
"How I Live Now" is in theaters now and on video on demand.