What is the story behind Sarah Phelps' twisty BBC detective drama Dublin Murders?

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Here's what you need to know...

Is Dublin Murders based on a true story?

No – and considering how many people get murdered or vanish into thin air in just the first few episodes, that's a very good thing.

The drama is based on the Dublin Murder Squad novels by Tana French, which began with 2007 mystery novel In the Woods. It's written for the small screen by Sarah Phelps, the serial adapter behind all those Agatha Christie Dramas and A Casual Vacancy.

And while these are places in Ireland called Knocknaree (in County Waterford and County Sligo), this particular setting – located "only a few miles from Dublin" – is totally fictional. The word itself means "hill of the king".

Which novel is Dublin Murders based on?

The drama actually tackles Tana French's first two novels in the six-part Dublin Murder Squad series: In the Woods (2007) and The Likeness (2008).

The author's debut novel In the Woods introduces us to Irish detective Rob Ryan (Killian Scott's character) and his partner Cassie Maddox (Sarah Greene's character), who must investigate the murder of 12-year-old Katy Devlin. But the case brings up old links to Rob's own past, and to the disappearance of children in the woods of Knocknaree more than two decades ago.

While Rob is the star of the first book, Cassie is the star of the second. In The Likeness, Cassie Maddox must find out the truth about the death of Lexie Madison, a young woman who appears to be her doppelganger.

But instead of adapting one book and then the other, screenwriter Sarah Phelps took a different approach. This way, both stories run alongside each other – and both detectives are under the spotlight.

"I mashed 'em," she says. "I plaited them. Because the first one is all about Rob, and Cassie is the baseline, and then it's all about Cassie and another detective is the baseline.

"And I thought, I don't want to do that. This is about a friendship, and this is about shared secrets and shared lies, and compromise and heartbreak and the dark places that we go to, and the things that scratch at the back of our skulls. And let's put them together so that the consequences are overwhelming for both of them at the same time.

"And when you say that out loud it sounds really easy. And then I went, 'Oh f**k it, now I've got to actually do it. Why didn't I do it in an easy way?' But I think it really works. Because Tana is a brilliant writer, and the books are very interior, they're really really strong interior monologues. You are absolutely in the landscape of these individuals, the hinterland, you're party to their every single thought. And it's how you make that exterior, how you take that interiority and make that interior landscape part of the world that we're in for everybody. So that's the challenge."

How was Dublin Murders adapted from the books?

Sarah Phelps was approached by production company Euston Films, who wondered if she'd take a look at the Dublin Murder Squad novels. Within a few pages, she was sold.

"The thing that's most important to me when I'm working on something, especially when you're going to make huge series which means you're going to be giving over like a years of your life and you're not going to get much sleep and you're going to lose contact with all your friends and everyone's going to hate you, is that you have to really, really love the source material," Phelps tells RadioTimes.com.

"There's something there which has to hook into your heart and make you want to do it. And the source material is amazing, she's an amazing writer, Tana. She writes brilliantly in a really layered way about loss and grief and memory and identity, and who we are and who we want to be, and why we are the way we are, and how we can't bear to look at that."

Phelps also found some other threads she wanted to pull out of the novels, which are set in Ireland from 2006 – and which take us back to the 1980s.

Those earlier scenes come before the arrival of the 'Celtic Tiger' in the mid-1990s as the Republic of Ireland suddenly hit a period of rapid economic growth. Jump forward, and the later scenes see us right on the cusp of the 2008 crisis and recession.

"What really struck me about her is that she was writing about a country, Ireland, going through a really volatile part of its life," Phelps says. "The the Tiger economy, the crash and then the recovery."

Phelps, whose mother is Irish and who just claimed her own Irish citizenship, says she felt an urge to write about "an Ireland which has suffered a diaspora from which it's still recovering, and to write a story about disappeared children in a country where lots of children disappear.

"And at the same time as I was starting to think about it, they had the stories about the children at Tuam, which is where they basically lifted up the septic tank and found babies bodies down there. And i just got this little thought in my head, don't go digging in Ireland you don't know what's there. And those were the kind of thoughts that just powered me when I was writing about it."

As she thought about how to adapt these novels, Phelps also turned to folklore and fairy tales.

"I kept thinking actually all of these stories actually are a version of a really, really old fairy tale, the children who go under the hill... if you go and do something, you'll go under the hill with the old ones. The old ones will come and take you," she explains.

"And you think, what were these old stories actually for? What these stories are actually for is actually to process what happened to those children, 'Oh yeah they went under the hill with the fairies.' Perhaps it covers up infanticide or something like that. But the genesis of those old fairy tales is always really, really dark. Cinderella is actually a really dark tale which is actually about people mutilating their feet to fit a glass slipper. I love all of that.

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"I love all of those Angela Carter [style] investigations into the darkness of how our imaginations are formed and how we tell each other stories and why we tell each other stories. What is the story going to do? It's going to keep back the beast. We're going to sit round the fire and we're going to tell a story and the beast is going to patrol just at the very edges of the light, and just scratch at the backs of our imaginations."

Then there are some classic Sarah Phelps-ian themes which creep their way in to her work, infusing everything with a certain unsettling atmosphere. "I think I'm always sort of writing in some way about political chaos, and deception and lies and conspiracy and paranoia and delusion and hubris and corruption in some way," she says.

Phelps has adapted novels by authors both living (JK Rowling) and dead (Charles Dickens, Agatha Christie), and each time she takes on a new project she puts her own spin on the story. Her adaptations have attracted critical acclaim and plenty of fans – but, of course, there are also outpourings of fury from certain viewers.

Is she worried about this as she tackles another bestseller – this time from Tana French? Absolutely not.

"If you consider what people are going to say before you've written, don't do it," she warns, "Don't. You can't. You'd never get anything done, and in a way you've got to be in such a bell jar when you're writing, and you've got to think about the characters and the story. What is this story that you're telling? You can't imagine what people are going to say because you don't know what they're going to say. You can't imagine it.

"Of course you're writing for an audience but ultimately sometimes, I know it sounds really selfish and really bad but I have to think, who am I writing for? I am the audience. Am I moved, am I laughing, am I crying? Do I believe what I've written? Do I believe it, and why have I chosen this word? Is this word the right word? Is this image the right image? Is this atmosphere the right atmosphere? How do I honour them, the people in my head? Because they're the most important.

"Because if I think about all the other people, everybody who may or may not watch it, and may or may not hate it, or like it or love it or loathe it or loathe me or whatever – but who's this person here, the one that's living in my head, the one that's speaking in the page? Them, it's them I think about."

Still, Phelps can understand why some people object so strongly when the TV drama doesn't fit with the way they read the novel, and with the pictures they already have in their heads.

"I don't think I'd ever police anybody else's version," she says. "But you do fall in love, and you get this profound sense of ownership. 'Don't anybody take my experience away from me. And don't anybody invalidate my experience and what I felt when I read this.' So it becomes an incredibly complex, involved emotional experience to unravel. You're not just saying this is about a book. People are hugely emotionally invested. 'This is my Poirot, what have you done! That's my Maigret, what have you done!'

"And it's just like, yes, but none of us read the same way. Your experience of reading it is not my experience of reading it. You read it and see this, and I read it and see that over there and that's the thread I'm going to pull. So it's a very kind of incredibly volatile and complex thing."

She adds: "When it's a book, it's yours, that's it. When it's on the TV, it's suddenly wrested out of your control... you can feel powerless. And people are very defensive when they feel powerless, and they feel that what they had to offer – in terms of their vision – has been ignored in favour of your one. 'Why is yours being validated and not mine?' It's a really interesting phenomenon I think. Really interesting."

Dublin Murders continues on Mondays and Tuesdays at 9pm on BBC1