Blanchett, most recently seen in 2008's "The Curious Case of Benjamin Button" will join the cast which begins shooting in Germany next week.
Saoirse plays the "Hanna" of the title, a 14-year-old girl from Eastern Europe who has been trained as an assassin by her father.
Cate will play the mother of a French girl who Saoirse befriends.
The plot calls for Saoirse to take a stand against the life her father has built for her against the life that Cate and her family offer.
Cate of course is very popular with Irish audiences after her pitch-perfect portrayal of murdered Irish reporter Veronica Guerin in the 2003 movie of the same name.