Her idea for the book came from two sources. She has a daughter aged 14 and a 12-year-old son, and when she met for coffee with a friend who has an older teenage boy, she became interested in the different worries they each had for their children.

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“Social norms are changing fast,” she says. “You can worry that a young man navigating the world might make a mistake that could be incredibly destructive.“

The second source was conversations about coercive control. “We have had horrific cases where things reach their absolute worst, say, a man kills his wife, and people say ‘It was a moment of madness, he was a good man’.” That’s simply not accurate, she says, but people are trying to make sense of what happened.

Another impetus was social media. She’s fascinated by the growth of true-crime documentaries and podcasts. “It’s absolutely exploded over the last few years. People are using the same narrative techniques that novelists use to draw people in and send them in this and that direction. And we’re failing to fully grasp the impact.“

Social media posts can be “absolutely lethal”. An example is the Facebook posts that surfaced after four-year-old Cleo Smith was abducted from a Western Australian campsite. “Tens of thousands of people were following accusations of all sorts of things. They feed off each other.” (Cleo is alive and well and her abductor is now serving 13 years in jail.)

In McTiernan’s book, Simon’s father hires a PR firm to cast social media suspicions on Nina’s parents. Do such firms exist? “Yes, absolutely. One was accused of doing illegal things. Someone accused of murder hired a reputation manager. I’m taking a bit of licence, but I don’t think it’s far from reality.”

When researching, McTiernan’s legal background comes in handy. She grew up in County Cork, Ireland, and practised as a lawyer for 12 years. But then the GFC struck. “We went from boom to bust. I worked for a couple of property developers who lost everything. It was incredibly traumatic, there were suicides. I just never wanted to practise law again, I was so burnt out.”

With a two-year-old daughter and a baby on the way, McTiernan and her husband Kenny, a civil engineer, started a new life in Australia. She went back to work in a quasi legal role but wasn’t enjoying it. She thought about doing an MBA but decided that instead of studying, she’d try to write the kind of book she liked to read.

“We were living in a tiny rental house, the electricity wasn’t great, there was no room for anything. But at 8pm when the kids were in bed I pushed the laundry aside, set up the laptop and worked every night except Thursday, when Kenny and I would open a bottle of wine.

“I knew almost from the beginning that I’d never stop doing this. This was the piece that was missing. I wrote what became The Ruin, but I was a beginner, and I was slow. I tried to get 500 words out a day and it was just terrible. But I didn’t know why.“

Gradually she began to analyse books by writers she loved, such as Tana French. She sent out some work to an agent. And finally came the surreal moment in the car park.

Today, McTiernan writes when her children are at school. She feels well, mostly. The brain tumour is long gone but has left a few effects. “I’m a miracle of modern medicine, propped up by the hormones I take through the day.“

She aims to get a first draft done in 12 weeks. “Then I use it to find the story, where’s the good stuff? I start again, cutting and rewriting.“

Disney FX is developing a TV series based on her thriller The Murder Rule and there’s a long-standing plan to film The Ruin with a production company headed by Irish actor Colin Farrell, though McTiernan’s not sure where that project has got to. Meanwhile, Cormac Reilly fans are hoping for a new book in the series.

“I can feel a Cormac book coming, but I have more ideas than I have time,” McTiernan says. What’s the secret of Cormac’s popularity? She based him on her husband Kenny.

“I wanted to write about a decent guy, emotionally literate. I love Rebus (Ian Rankin’s cop hero), but I’m reading he’s estranged from his daughter and feeling sorry for himself, and all I’m thinking is ‘You could call her’.”

What Happened to Nina? is published by HarperCollins at $34.99. On Monday at 7pm Dervla McTiernan discusses the book at the NSW State Library, 1 Shakespeare Place, Sydney. Details: here. She talks about it with Jane Harper at Cinema Nova, 380 Lygon Street, Carlton on March 6. Details: here.

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