Farrell, 37, and Weisz, 43, join the previously announced Ben Whishaw, Lea Seydoux, Olivia Colman, Ariane Labed and Aggeliki Papoulia, reported Ace Showbiz.
It will mark the English-language directorial project by Oscar-nominated Greek director Yorgos Lanthimos.
Described as an unconventional love story, it is set in future where finding a partner is a matter of life or death.
Lanthimos co-writes the script with his longtime collaborator, Efthymis Filippou.
Farrell was last seen in 'Saving Mr Banks' with Emma Thompson and Tom Hanks. He will next appear in Akiva Goldsman's 'Winter's Tale'.