Ronan, a four-times Oscar nominee for Atonement, Brooklyn, Lady Bird and Little Women, will be shooting The Outrun this summer.
Orkney-born Amy Liptrot, whose book recalls her homecoming when she was 30, is working with German director and screenwriter Nora Fingscheidt on the film adaptation.
The Outrun, which was described as "bold-hearted and brave-minded" by The Scotsman when it was published in 2016, has since sold more than 110,000 copies in the UK.
Liptrot, who was brought up on a sheep farm, wrote The Outrun over two winters in a cottage on Papa Westray.
It recalls how she began drinking at the age of 15, her hedonistic lifestyle in London, where she moved to pursue a career as a music journalist, her treatment in rehab, and her new life after moving back to Orkney.
Liptrot’s book evolved from a blog she started writing in rehab and nature columns she started working on in Orkney.
Recalling writing of her debut book, she said: “There were gales and power cuts and high seas that cast spindrift across the whole island.
"There were meteor showers and northern lights. I watched fireworks on other islands and heard the calls of wild geese high above. I swam in the cold ocean and dyed my hair a furious blue.
"At night I sat by the fire with a blanket over my knee and my glowing laptop and kept in touch with friends back in London – the digital ghosts of my old life.”
An announcement on the adaptation states: “Ronan stars as Rona, who fresh out of rehab, returns to the wild Orkney Islands after more than a decade away.
"As she reconnects with the dramatic landscape where she grew up, memories of her childhood merge with the more recent challenging events that have set her on the path to recovery.”
The Outrun will be the Irish film star's second major feature film to be made in Scotland following Mary Queen of Scots.
Ronan said: “It was Amy’s voice that first drew me to The Outrun.
"Her unusual way of seeing things in a way that perhaps you only can when you’ve been to the darkest place you can go within yourself.
"I have been waiting to play a part like this — the messiness, hopefulness, dreaminess, authenticity and humour we want to achieve — I’ve been scared of it, but with Nora I feel ready.”
Liptrot said: “I’ve been working with brilliant director-writer Nora Fingsheidt and hotshot producer Sarah Brocklehurst on an adaptation.
"We’re thrilled to have Saoirse Ronan onboard and the are looking forward to filming in Orkney this summer.”
Fingscheidt said: “Amy’s intriguing and poetic story shows how one can find their lives broken into pieces, and that recovery can happen in the most unexpected of ways and places.
“There are wonderful gifts already in place to make this a powerful and immersive film. One is the setting, the vast and cinematic landscapes of the Orkney islands; the other is our main actress, who brings magic to the screen with her unique mix of charisma and emotional depth.”