The 19-year-old actress was rumoured to be starring in the three part fantasy adventure, directed by Peter Jackson - whom she had previously worked with on 'The Lovely Bones' - but decided it would be better to focus on other projects.
She said:"There was talks about me playing the wood elf Itaril but I had to turn it down. I was really disappointed but there were other projects I had to consider and to spend a year doing 'The Hobbit' wouldn't have left me time for anything else."
The Irish star is currently filming Ryan Gosling's 'How to Catch a Monster, which she is finding "a bit twisted", and also has a string of other films in the pipeline.
She added to the Daily Mirror newspaper: "I'm filming 'How to Catch a Monster', which is Ryan Gosling's writing and directing debut and it's cool, dark and a bit twisted, too. It's about an underwater town, and the struggles of the characters living in this dilapidated place. And I'm in 'Violet & Daisy', in which I play a hit girl.
"'How I live Now' and 'The Grand Budapest Hotel' are all coming out in the next year. Then I'm due to play Mary Queen of Scots in a movie about her starting soon."