She exclusively told BANG Showbiz: "When I was younger, I was about 11, I had an eating disorder. I was very tormented and, eventually I was just like, 'I hate this, I don't want to be this person any more. My 'Harry Potter' character Luna really inspired me, I thought, 'I really wanna be someone like that.'
"It shaped me. I had to learn how to channel it. Those eating disorders are so much about you wanting to be perfect and be the perfect size, but I also had to learn it was a very cowardly thing to strive to be a certain shape."
Instead, the Irish-born beauty – who now lives in Los Angeles – channeled her energy into being creative.
She added: "What I learnt was that you could put that into creativity. You could put that fierceness, fire and ambition into something creative. It just takes a lot more guts. It's not certain, whereas when you are trying to lose weight it's certain you can keep getting smaller whereas with art, it may fail.
"So it's definitely a braver thing to do. It's a good energy to have, and a good motivation."
Tickets for Warner Bros. Studio Tour London - The Making of Harry Potter are on sale now at www.wbstudiotour.co.uk and have to PRE-booked in advance.