Saoirse Ronan has said that she is not confirmed to work with Peter Jackson again in the Hobbit movies. Ronan says that the rumours are just that but that she would jump at the chance to work with Jackson and Walsh again (she previously filmed The Lovely Bones with Peter Jackson and Fran Walsh).
The video here kinda makes it look like she's not giving everything away but that's probably simply due to loyalty to Jackson and Walsh. I wouldn't bet against her working with them again.
The Hobbit is alien to me so i wouldn't even have a clue if there are any or many young female characters in the book for her to play.