The Oscar-nominated Irish actress's involvement in the two-part fantasy prequel was confirmed by the movie's casting director Ros Hubbard.
Asked about the strong presence of Irish acting talent in The Hobbit, which features Aidan Turner, James Nesbitt and Ronan, Hubbard told IFTN: "We had been wanting to put Aidan Turner into film for a long time because he had done an awful lot of TV and we told him when we finally met him, 'We are going to concentrate on getting you into the movies.' I didn't think it would be as big as this. But how great for him.
"And Jimmy 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."
Ronan's other movie roles include Atonement and Peter Weir's The Way Back.
Filming on The Hobbit was recently delayed after Jackson underwent surgery for a perforated ulcer.
> Click here to watch our recent interview with Saoirse Ronan