Farrell, 39, will play Graves, a wizard that Newt Scamander (Redmayne) meets in New York, said the Hollywood Reporter.
The story centers on Newt's adventures in the secret community of witches and wizards as he attempts to document magical creatures. The events of Fantastic Beasts take place 70 years before Harry Potter reads Newt's book in school.
Dan Fogler also was recently cast in the film.
David Yates is directing the return to JK Rowling's wizarding world, and Rowling wrote the script.
The film is set for release on November 18, 2016.
Farrell is currently starring on HBO's second season of "True Detective".
