The Irish actress was in talks to reunite with her The Lovely Bones director Peter Jackson for the two-part project, which is currently filming in New Zealand, but Ronan has decided to pass on the offer.
She tells Digital Spy, "I won't be able to do The Hobbit unfortunately. I would have been tied up for quite a long period of time and there's a lot of projects in the meantime that are also really interesting and I'd like to be a part of."
But Ronan can't wait to see Jackson's take on J. R. R. Tolkien's novel, adding, "I went on to (Jackson's) blog, they sent me the video. They did their first video a few weeks ago so I'm going to keep myself updated on it."