Last time I interviewed Cecelia Ahern, a man with a sub-machine gun waited in the wings. Cecelia was not alone; she was with her sister, Georgina and her father, the then Taoiseach, Bertie Ahern. That was in 2005 when Bertie was top of the heap, Georgina was famous for being married to Westlife's Nicky and 24-year-old Cecelia was on the way to becoming a literary star on the back of her debut novel, PS, I Love You

Today, 16 books and more than 25 million sales later, Cecelia is arguably more famous than her father, certainly more widely read than Westlife and storming Hollywood (Samantha Who? will be followed by a TV adaptation of Roar, her 2018 collection of short stories, produced by Nicole Kidman). Now with her latest novel, Postscript, a PS to PS, I Love You, Ahern returns to the mother lode.

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When we met last month there was no man with a gun. Just a beaming, heavily pregnant Cecelia (she gave birth to her third child, Blossom on October 5) and her publicist. The author was bashing out interviews ahead of maternity leave and just before we met, she revealed that Postscript was going to be made into a Hollywood movie. "I'd known about it since June but couldn’t confirm," she says. As yet, there is no screenwriter or director is attached but the star of PS, I Love You, Oscar-winner Hilary Swank is on board. "Hilary contacted me when we first announced Postscript and said she wanted to read the book," she says. "She wanted to see what would happen next. So I sent her the book, she devoured it and said 'I’m in!’"

She says Postscript was the most challenging book she has written. "I thought about writing the sequel to PS for years," she says. "I came up with the idea in 2012 (following the birth of her second child, Sonny) and wrote a chapter privately for myself. I spoke with my publishers to see if they were interested. I analysed this more than any other book I have written and ultimately, the question was: ‘Am I excited at the thought of writing this story?’ The answer was a definite yes. I wanted it to be a strong stand-alone novel regardless of whether people read PS, I Love You or not." (The book is dedicated to fans of PS worldwide). Set some seven years after the events of PS, its central character is again Holly Kennedy, now in a new relationship but her past is hard to escape.

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Ahern was just 14 when she attempted her first book, Beans on Toast and a Bottle of Beer. She laughs. "I have it at home somewhere but I never did finish it. My mom told my English teacher that I had been writing it and, as a result, he kept asking when he could read it. I was so embarrassed. And I never did finish it."

Through her teenage years, she dabbled in prose and poetry but her main writing was in diaries. "I was always documenting how I was feeling, so I wrote my diary every day. I’d have my shower in the evening, go to my room, write down everything I was feeling and go to bed. Writing was like a release, getting everything out of my head. And whatever fiction I wrote I didn’t share with anybody."

She wrote her debut novel, PS I Love You, when she was 21. "I had graduated from college, I was living at home, I had no job and I had no money," she says. "Then I wrote that book and everything changed. I suddenly had a career and was travelling the world and had responsibilities and deadlines. It changed me too. I had to adapt."

Yet you didn’t go bananas? "It was a two-book deal so I had to get my head down. I don’t understand how you can go bananas but then I’m not that kind of person. In any case, writing has always been my coping mechanism, so I was more than happy to just put my head down to cope with the craziness that was going on."

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P.S. I Love You told the story of a terminally ill man, Gerry, who leaves 10 letters for his wife, Holly, each one signed off with the line ‘PS, I Love You’. It was a novel idea, one that made the book a global bestseller with the spin-off film a box-office winner. "I’m guessing (the idea) came from where I was at that time," she says. "I was 21 and feeling quite fearful of losing the people that I loved. Obviously, I was drawn to writing as well and anything hand-written is very special, it captures a part of a person, their spirit." When was the last time she wrote a letter? "Not sure. It’s mostly notes in cards. But I love when my parents write my kids birthday cards. I love seeing their writing in a card. For me, that’s them."

Does she have a favourite letter? "Maybe my parents or my husband? No, it has to be my kids.! When they learned to write it was so moving seeing their character in their writing. I found this in my pocket today (She searches about in her coat pocket and produces a piece of paper). This is a little note from my son." She unfolds the sheet and reads from it. "’Hi mum and dad! From Sonny’, and there’s a picture which is the Eiffel Tower (she laughs). Now that to me means the world. I will carry that me all day."

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Such felicities propel Postscript but you’re occasionally broadsided by a profundity like: ‘Death catches us when we fall.’ "I remember the moment I wrote that line," says Ahern. "It was almost like an epiphany where I thought ‘Oh we think of death as the grim reaper but in this instance was almost like the carer, taking you away from suffering."

There are also clues to the author, for the most parts a private person. In Postscript Holly states: ‘Perhaps we are better off cleaning up our messes alone.’ It’s something Ahern also believes in. "Yes, I’m a bit of a lone ranger alright," she says and laughs. "I ask for help if I need it but that is rare. I also don’t self-analyse and the only time I think about myself is when I’m doing interviews and that can be disturbing." Isn’t writing a type of self-analysis? "Not for me," she says. "When I wrote Flawed, my first Young Adult novel, I wrote it in the character of a teenage girl at school. It was only afterwards, when I was doing interviews and people were asking me what I was like as a teenager at school, I had to think because I did not go back into my own teenage years to write that book. It was not about me."

And yet there are similarities, slippages between real life and fiction. "I have an obsession with time," says Ahern. It’s a theme that threads through Postscript. "Also I’m a planner (nearly every January, she starts a new book, a work ethic that churned out 15 titles in 14 years). And while PS I Love You was telling the story from the bereaved’s perspective, Postscript tells it from the perspective of people who are running out of time: How do they prepare to say goodbye and prepare the people they are leaving behind for a world without them? That is something I definitely think about. When my kids were babies and I was travelling a lot I’d write them a letter before I’d get on the plane just in case anything happened to me."

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In Postscript, Ahern posits that no matter what age you are you always revert to a childhood version of yourself with family members. "That is so true," she says and laughs. "My husband (Barry) will always know when my dad calls me because I’ll be like ‘Oh hi dad!’ sounding like I’m five years old." There is also a joke in Postscript (What do you call a zoo with only one dog? A Shih Tzu) that is a favourite of Cecelia’s daughter, Robin. So what’s Cecelia favourite funny? "I’m terrible at jokes. I can’t tell stories, I can’t tell jokes and I’m also not too good at listening (laughs). So I tell a story using the shortest amount of words possible. It’s like ‘this thing happened, blah, blah, blah and it’s over. I suppose that’s sort of funny as I make my living from telling stories."

Ahern only follows six people on Twitter. Apart from family, the other two are Elon Musk and NASA. Space has been an obsession ever since childhood when V and Alien Nation were among her favourite TV shows. "There is definitely more out there than we know," she says. "And if these people are exploring the universe I want to know about it. If NASA discovers something new in space, that is real news. In the past, people laughed that there was alien life out there but there must be some sort of life form out there somewhere." Would she go into space? "No, because I like having my feet on earth but I’m fascinated by what’s out there. And my daughter wants to go to Area 51. She has inherited that from me!"

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The week before she gave birth to baby Blossom, Cecelia celebrated her 38th birthday. "I’ve barely thought about it because there has been so much going on," she says. With a complete draft of her 18th novel in the bag ("I will edit it in May"), the next few months will be all about time out with family. But I suspect she won’t be entirely taking the foot off the gas, an independent woman with a strong work ethic.

"I believe that we are mostly responsible for the situations we find ourselves in and really that’s the nature of what all my books are about," she says. "Life can throw things at you but it’s up to you how to deal with those situations."

Postscript by Cecelia Ahern is published by Harper Collins.