According to Deadline, Lost‘s Maggie Grace has been cast to play Irina in Breaking Dawn!  Here is a description of her character (WARNING: SPOILER IF YOU’VE NOT READ BREAKING DAWN):

Irina is a member of the Denali coven, considered cousins to the Cullen clan as the only other “vegetarian” vampire group. When Irina blames the Cullens for the death of her lover, her actions set in motion a terrifying chain of events.