Unfaithful focuses on two couples, one younger, the other older, who are woken from their domestic torpor when they are forced to come to terms with the consequences of different kinds of betrayal.
For McCafferty, wrestling for words as he sits on the sofa of the Traverse Theatre's Leith warehouse rehearsal space, it's not always easy to explain where his play came from.
"In the society we live in," McCafferty says in his staccato Belfast accent, "things like somebody being unfaithful, especially as seen through the media, looks like a very black and white world, whereas we all live in a far greyer area than that nowadays, and I wanted to write something to show that.
"If somebody was to be unfaithful, what comes out of that isn't necessarily a bad thing. It can have a good consequence. That's what we're looking at in the play.
"Everything in it is about being unfaithful, whether that's to do with a small lie or whether it's about having sex with someone you've just met. I'm not interested in the actual act. It's the aftermath.
"It's like putting sex or violence onstage. It looks stupid. It's what happens afterwards that counts."
Without giving too much away, in Unfaithful, this involves the older couple having a form of reawakening.
"It's asking what it means to be unfaithful," says McCafferty, "and how important that act of unfaithfulness is compared to others. This is taking things out of context, but you can be unfaithful just by talking to someone.
"It's how they deal with it. In relationships, people may make a tacit agreement not to talk about something, in the knowledge that if they do confront these things it all might kick off.
"Soap operas are all driven by the idea of being caught, and that being caught is the worst thing, but it's not, and in that way I suppose long-term things are more important than minor misdemeanours."
Volatile relationships and things left unsaid have been at the heart of McCafferty's work since he first came to prominence with Mojo Mickybo, a story about a Catholic boy and a Protestant boy who bond over Butch Cassidy and the Sundance Kid during the Troubles in 1970s Northern Ireland.
In Scotland, Theatre Jezebel produced McCafferty's adaptation of Days of Wine and Roses, JP Miller's booze-soaked TV play, which he relocated from America to 1960s London, where an Irish couple tear emotional chunks out of each other.
This time last year at the Traverse, a new play, Quietly, simmered its way through a bar-room confrontation that confronted Northern Ireland's prejudices past and present.
"I'd written Quietly and Unfaithful quite closely together, and they seemed to be similar," McCafferty says. "Both plays deal with the result of an act, and they're both deliberately quite short. I wanted to write something that was compact and complete. They are what they are."
McCafferty seems to have purged himself of this approach for his next piece. Death of A Comedian is set over four performances by a comedian who moves from being an edgy outsider to a commercial success.
"He becomes bland," says McCafferty. "I don't like to talk like this, because it makes things in my head sound more grandiose than they are, but that's about capitalism destroying whatever creative soul you have within you, and that's the price of success."
McCafferty pauses to reflect.
"I hadn't thought about this," he says, "but there seems to be a connection between all three plays.
"They're all about consequence and action. They're also saying that bad things don't necessarily lead to bad things, but also that you shouldn't think you can do something and escape the emotional consequences."
Where betrayal and infidelity onstage are too often the preserve of well-heeled literary types, McCafferty's world is occupied by characters with precious few economic safety nets and with only each other for comfort.
"I've never written about what we perceive to be important people," McCafferty says. "I think it's an absurdity that the higher up the scale you go that you think you know more. Why should we be led to believe that Tony Blair knows any more than us. It's a nonsense.
"In the job we do, to write this play, we can talk about infidelity, and we do in a way that other people don't. But instead of looking at it as a morality, we should infuse it with love. The play is about love. It's not about morality. Take the morality out of it, and just look at what happens and why people react the way they do."
Unfaithful, Traverse Theatre, to Aug 24.