The pair first collaborated on 2008's In Bruges, a dark comedy that earned writer and director McDonagh a Bafta award and an Oscar nomination, and Farrell a Golden Globe.
But, as it transpires, In Bruges was simply McDonagh's trial run at film-making.
"Just before we started In Bruges, you said, 'If you think In Bruges is good you should see the other script I wrote, it's much better'," recalls Farrell, laughing.
"He said, 'I'm only doing In Bruges to see if I like making films. And then I'll make the real one'."
Looking slightly sheepish, McDonagh puts his hands up and jokes: "That was all complete honesty.
"But seriously, Seven Psychopaths was always going to be a bigger script."
An accomplished playwright (he's the recipient of two Laurence Oliver awards and four Tony award nominations), McDonagh wrote the screenplay for Seven Psychopaths at the same time as In Bruges – but there was never any question as to which would be his feature film directorial debut.
"Seven Psychopaths was too big to get my head around cinematically before I dipped my toe in the water," explains 42-year-old McDonagh.
"As a first-time director, I decided to start with something that was more about things I knew."
A tale of two hitmen, In Bruges was in some ways a character and relationship study set in one place, "and I felt I knew that territory from my work in theatre", says McDonagh.
"Seven Psychopaths was like a puzzle, a gigantic cinematic jigsaw. I don't think I could ever have done this film without having done In Bruges first."
The title of their latest film is also the name of the screenplay which the movie's protagonist Marty, played by Farrell, is struggling to finish.
"Marty's fallen in love with this title but he hasn't come up with all the psychopaths," explains Farrell, 36, who's appeared in the likes of The Minority Report, Miami Vice and, most recently, a remake of Total Recall.
"While Marty's renowned for writing good dialogue and violent scenarios, he's trying to take that violence and somehow render a story that is, in essence, about peace and love."
The idea for the movie kicked in seven years ago when, in real life, McDonagh was inspired to write a story about a psychopath.
"Another couple of stories soon followed and it just kind of snowballed from there," he says of the intertwining tale.
But at its core he wanted to explore "the tug-of-war between wanting to do something spiritual but also something dark and deranged".
In the film, when we meet Marty he's already past his deadline and way past the end of his girlfriend Kaya's (Abbie Cornish) patience.
The situation isn't helped by the constant disruption his best friend Billy (Sam Rockwell) poses. That's never more apparent than when Billy and his dog-napping business partner Hans (Christopher Walken) discover a particular pooch they've swiped happens to be the beloved pet of notorious gangster Charlie Costello (Woody Harrelson).
As the three go on the run, with Shih Tzu Bonny in tow, Billy remains determined to help Marty finish his screenplay – at whatever the cost – and the result is a blood-soaked black comedy that's as funny as it is difficult to stomach at times.
"Every now and then, writing jumps off the page. This one does that," says Farrell.
Dressed in jeans and black jacket, he's wearing a long silver chain over a V-neck green T-shirt and a bangle that jingles as he moves.
While his tan might be all LA (he lives there most of the year), Farrell comes across as a down-to-earth Dublin boy who couldn't be more delighted to be working with McDonagh again.
"He has a particular way of stringing words together that has an insane effect on the imagination," says Farrell.
With seven distinct performances on the agenda, the challenge for McDonagh was to have a firm grip on what exactly defines a psychopath.
"Some of the characters have elements of the psychopathic to them but at the same time they don't. I guess psychopathy is in the eye of the beholder in some ways," muses McDonagh.
When Farrell first read the script, he admits he was drawn to the role of Billy, who McDonagh's described as "a mess but in a disturbingly likeable way".
Familiarity had Farrell lean in that direction – "because he's more like Ray, the character I played in In Bruges," he says. "Marty's more the observer, the only one that's sane really!"
No stranger to meshing the juxtaposition of humour and darkness in past projects, McDonagh admits his humour is "leavened with a little bit of darkness".
"But the trick is to never let the darkness weigh the humour down," he notes. "And I try to put a lot of humanity in my scripts. I think this one has a good amount of tenderness and is a big human story at its heart. It's really about friendship."