Ahern was born in Dublin, Ireland on September 30, 1981 where she still resides today. The daughter of Bertie Ahern, former prime minister of Ireland, she grew up in a beautiful suburban town in a large home, but her parents split when she was a young girl. Prior to her writing career, she was a part of an Irish pop band named Shimma, where they finished third in the Eurovision Song Contest in 2000 with their song titled "When You are Near". The music industry is still prominent in Aherns life today as her older sister married Nicky Byrne who is a member of the Irish pop group, Westlife. She earned her bachelor's degree in Journalism and Media Communications from Griffith College of Dublin, but left school to become a writer before completing her masters degree. In 2010 she married David Keoghan and they have two daughters and one son. She lives a private life now, as she is not incredibly active on social media and she avoids bringing her personal life into the public eye as much as possible.
Ahern wrote her first novel at the age of 21 in 2003, titled P.S. I Love You, which remained the number 1 bestseller in Ireland for 19 weeks, also hitting number one in the United States, United Kingdom, Germany, Netherlands, and Ireland. This book became so popular, it was adapted into a movie in 2007 where Ahern played a key role in rewriting the adaptation of her book onto a screen. Her next book also hit number one in Ireland, titled Where Rainbows End, commonly known as Love, Rosie in the United States, also adapted into a movie in 2014. Her books are published in nearly 50 countries and she’s sold 25 million copies worldwide.. Her titles Thanks for the Memories and The Time of My Life were adapted into German TV series' as well. Her works have been loved worldwide by many fans because she explores sensitive topics in her writings that make her books relatable but also heartwarming to the reader.
Ahern won her first two awards in 2005: Corine Prize for Fur Immer Viellecht for her novel Where Rainbows End, and the Irish Post Award for Literature. In 2007 she earned her next two awards: the Cosmopolitan US Fun Fearless Fiction Award for her novel If You Could See Me Now and the Glamour UK Writer of the Year Award. Ahern won Irish Tatler Woman of the Year Awards Writer of the Year in 2009 as well as in 2016. For her novel The Year I Met You she won the Bord Gais Energy Irish Book Award for Popular Book of the Year in 2014. Most recently she has won the Redbridge Teenage Book Award in 2017 for her young adult novel Flawed.
Ahern began writing screenplays in 2007 because she believed "novels [aren't] always the correct way to tell the story". In some instances Ahern felt the only way to truly give a story the proper interpretation it deserves was to create a visual representation, not just words on a page. She was a writer on the ABC comedy series titled "Samantha Who?" and the Apple TV series "Roar" which was an adaptation of several of her short stories in her book titled Roar. Ahern said she enjoyed writing for TV because it allowed more collaboration on the work, but she disliked the lack of personal expression that came with the collaboration. In all of the novels she wrote that were adapted into TV and film, Ahern played a crucial part in creating the story to be as accurate to her books as possible.
Ahern has always stated she has a fascination with the human spirit and she enjoys writing books about overcoming grief. Several of her most famous titles follow along the story of someone grieving a loved one. She believes that in order to write about someone, you need to understand the emotions behind the character and what causes them to feel that way. In an interview she once stated she loves the month of January because it signifies new beginnings. Most people dislike January because it's dark, cold, and there's a lot of pressure after the new year. Ahern disagrees with this point of view because all these attributes of the month of January cause growth for people. Most of the novels she has written have been considered adult fiction, within the past ten years she has transitioned her writing to be more tailored to young adult audiences, beginning with her book Flawed. She finds it important to explore important messages with young readers and really get into the mind of the characters she writes about. Growing up she saw reading as an escape from reality, and that has caused her to want to inspire young readers the same way she was as a young girl.