Hanrahan was one of the game designers included in the experimental community connected to the Gaming Outpost.[1]: 406 Hanrahan oversaw the writing for Paranoia when Mongoose Publishing brought it in-house.[1]: 398 Hanrahan wrote the fifth RuneQuest setting from Mongoose, Hawkmoon: The Roleplaying Game (2007).[1]: 399 Hanrahan wrote the Traveller Core Rulebook (2008), which was able to outsell RuneQuest and become the new #1 game from Mongoose.[1]: 401 Hanrahan wrote the 12th-century setting Deus Vult (2010) which became one of the settings that received new support in RuneQuest II.[1]: 399 Hanrahan was one of the employees let go in the layoffs which resulted when Mongoose separated from Rebellion in March 2010.[1]: 402
Hanrahan is the author of the Paranoia novel Reality Optional.[4] He has also contributed short stories to the Stone Skin Press anthologies The Lion and the Aardvark[5] and Schemers.[6]
He released his debut novel in 2019, and its sequels in the "Black Iron Legacy" series in 2020 and 2021:
The series is a gothic/steampunk adventure, set in the city of Guerdon, which has been described as being inspired by Hanrahan's native city of Cork.[10]
^Ltd, Cubicle 7. Entertainment; Ryder-Hanrahan, Gareth; Nepitello, Francesco (2016). The Heart of the Wild: Gareth Ryder-Hanrahan. ISBN978-0857441430.{{cite book}}: CS1 maint: numeric names: authors list (link)
^Ian Sturrock, Paul Tucker, Harvey Barker, Vincent Darlage and Gareth Hanrahan, Conan: The Roleplaying Game - Second Edition, Mongoose Publishing, Swindon (England), 2nd Edition: September 2007, cover art by Chris Quilliams, map by Clayton Bunce, inner illustrations by Andrew J. Hepworth, Warren Mahy, Danilo Moretti, Chris Quilliams, Jason Rosenstock, Chad Sergesketter, Ronald Smith, Vebjorn Strommen and Alejandro Villén Real, hardcover, 1.4 x 8.3 x 10.8 inches, 424 p., ISBN978-1905850068