Saoirse Ronan was actually rumored to have been headlining the new Star Wars film many months ago, but it looks like that bit of information didn't pan out. Even though she confirmed that she had read for the project, she also added that most of young Hollywood had also been up for some part at one point or another. Considering the buzz that's surrounded the movie for this past year, that's not surprising. What is surprising is the fact that the filmmakers went for mostly unknowns for the film's lead cast, although it's probably a smart decision in the long run. This way, they'll be able to keep the film fresh for the audience, without any burden that established stars would bring.
So far, we know that Daisy Ridley, John Boyega, Domhnall Gleeson, Andy Serkis, Oscar Isaac, and Adam Driver are part of the cast. However, J.J. Abrams recently announced that they hadn't yet completed casting, and were still fine tuning the script. This makes sense because before the official announcement of the principal cast, there were rumors that there were additional characters whose roles hadn't yet been filled.
Both Lupita Nyong'o and theater actress Maisie Richardson-Sellers were also rumored for a major role, and rumors of Saoirse Ronan's involvement haven't died down either. According to sources, there are still several major roles that haven't yet been filled, including two prominent female roles. It looks like Lupita and Maisie might be vying for one role, and the other might be up for grabs. Saoirse's schedule is pretty open, so her joining the film at this late stage is not out of the possibility. She is filming a movie called 'Brooklyn' right now, but it's with Domhnall Gleeson, a confirmed Star Wars cast member. Perhaps he could put in a good word for her?