Brennan was born in Ireland. She has lived in New York. She obtained a Creative Writing MA and worked as a librarian in Surrey, England, before moving to Dublin where she currently lives.

Brennan has been writing since the age of five. She has also written fanfiction, including slash, under the pseudonym 'Maya', for the Harry Potter fandom. She is considered a Big Name Fan. All of Brennan's fan works were made unavailable to the general public in September 2008. She has a blog on Livejournal which has over 4000 subscribers.

She wrote her first novel, The Demon's Lexicon, while studying for her Creative Writing MA. The publishing house Simon & Schuster obtained a three-book contract deal with her which involved an undisclosed six-figure sum.