The Irish actor plays an ex-convict who falls in love with Knightley's reclusive actress character in the upcoming crime movie.
Farrell admits he is in awe of the Pirates of the Caribbean actress - and he's determined to appear onscreen with her again.
He tells Britain's Live From Studio Five, "She is gorgeous. She's really gorgeous and lovely - a lovely woman and incredibly smart and bright and talented and creative and generous. She was lovely, I mean she really was, so it was easy working with her.
"I was a really big fan of her work beforehand and even more of a fan of her work afterwards. I wish I got to work more with her, I really did. I felt like it was kind of just a rehearsal for maybe some other thing that maybe I'd be lucky enough to get the chance to do with her at some stage."