For a couple of months now, Saoirse Ronan has been rumoured for The Hobbit films, but it's been difficult to know whether this was just assumption based on the fact she was in Peter Jackson's Lovely Bones, or if there was real substance to it. However it appears it may well be true, as according to an interview for IFTN with Hobbit casting director Ros Hubbard, she has indeed been cast.
There's no news on her role (although Itaril has been mentioned), but when asked about the presence of Irish cast members Aidan Turner, James Nesbitt and Saoirse Ronan, Hubbard says "Jimmy [Nesbitt] is thrilled to bits, his whole family have gone out, it is just wonderful. And working with Peter Jackson is like working with a family. So they’ll have a great time. Saoirse’s family will go too, everyone is very close and very loving on those sorts of jobs. It’s not like typical studio movies at all."
It's implicit implication Ronan will be in the movie rather than straight out saying it, but it's close enough.
The Hobbit movies start shooting soon (once Peter Jackson has recovered from surgery on a perforated ulcer) for release in 2012 and 2013.