The star returned home to film the movie, an adaptation of August Strindberg's thrilling play of the same name, about the daughter of an Anglo-Irish aristocrat who is seduced by her father's valet, played by Farrell.
The love affair soon turns dark, and the actor admits he found it draining to play such a controlling character.
He tells the Associated Press, "It's tough, as tough as this job can be. It was the toughest time I have had in 15 years by a mile. It was hard, it was brutal.
"The behaviour and the sustained cruelty and trauma are just brutal. It was cool, it was interesting but it was tough. I was glad to be done with it, like a few others. It was very challenging but it will be interesting to see what it becomes."
Jessica Chastain plays the titular character in the film, which will be released in 2014.