[7] O'Shaughnessy bought his first typewriter when he was fourteen and wrote many short stories, comic scripts, and books that he never finished, many of which are unpublished.

His first success came to him at age fifteen, when he was a runner-up in a TV script-writing competition for RTÉ in Ireland, with a dark comedy story entitled A Day in the Morgue.

[8] After university, O'Shaughnessy worked in a TV cable company in Limerick for two years, before becoming a full-time writer at the age of 23.

The series was a huge global success and by 2021 his books were on sale in 40 countries, in 32 languages, and had sold thirty million copies worldwide.

[10] O'Shaughnessy continues to live in Pallaskenry in Limerick, Ireland with his wife Helen Basini, a freelance university lecturer and global aid specialist.

[13][14] This is a four-book series covering 200 years in the life of Larten Crepsley, a vampire first introduced to readers in The Saga of Darren Shan.

However, in March 2008, Ayuamarca was re-published as Procession of the Dead (with substantial changes) by the Voyager imprint of HarperCollins (O'Shaughnessy's children's publisher).

On 1 April 2020, O'Shaughnessy announced that he had silently released three books digitally due to the COVID-19 pandemic, making up the first volume of the Archibald Lox series.

Inspired in part by The Adventures of Huckleberry Finn and based in a brutal, fantastical world, it details the perilous quest of Jebel, the thin and scrawny son of a respected executioner, to petition a fire god for invincibility, so that he can succeed his father.

O'Shaughnessy tweaked and slightly expanded it ten years later, and it was released as a book by the publishing group Barrington Stoke with illustrations by Zack McLaughlin.

O'Shaughnessy has not revealed details of any of his unpublished books, with two exceptions — Mute Pursuit and The Cannibal King: Universal Studios bought the film rights to the first three books of The Saga of Darren Shan and combined them to make a single film, Cirque du Freak: The Vampire's Assistant, released in the US on 23 October 2009.

[24] In March 2021, Susan E. Connolly was announced to be attached as the lead writer on the project, with The Electric Shadow Company releasing a teaser image depicting a blood-spattered B Smith opposite her best friend Vinyl to their website.