The actor has served as the special guest of the night twice before, but it has been a while.
The 23-year-old Ronan can now be seen in the Greta Gerwig-directed Lady Bird, which hit theaters in early November. This will be Ronan's hosting debut on SNL as well as a great opportunity for her to plug her acclaimed new movie Lady Bird.
Those SNL episodes will round out December's three-week run of new shows. This visit is to promote their 14th studio album, "Songs of Experience", out December 1.
Franco will be the first guest to host Saturday Night Live this December.
Set to air on December 16, the final "SNL" of 2017 will feature host Kevin Hart and musical guest Foo Fighters.
December 2 will bring a veteran musical guest and a first-time host.