The pair will reunite for the as-yet-untitled film version of the bestselling book by Joanna Molloy and John Donohue, according to Variety. The story is based on Donohue's attempts to find the friends he met while fighting in the Vietnam War.
Brian Currie and Pete Jones will co-write the screenplay with Farrelly.
Mortensen and Farrelly's last collaboration, "Green Book", was awarded the Best Picture Oscar in 2019.
Meanwhile, Viggo's first attempt at filmmaking, "Falling", got a big thumbs up last week when it was named among the 56 movies selected for Official Selection at the cancelled 2020 Cannes Film Festival.