The Hollywood Reporter describes the character as the love interest of Charles Xavier, aka young Professor X (James McAvoy), which might contradict plot rumors involving a love triangle between Xavier, young Erik Lensherr/Magneto (Michael Fassbender) and Emma Frost (Alice Eve). Of course, it's possible that Charles and Moira get together later in the story.
Either way, Byrne, who's also a regular on the cable series Damages and has co-starred in the films Knowing, Sunshine and 28 Weeks Later, will likely be one of the more prominent players in what's threatening to be an over-filled superhero movie. At least she gets to act alongside an amazing ensemble of talent, which also includes Jennifer Lawrence (as Mystique), Nicholas Hoult (as Beast) and Kevin Bacon (Sebastian Shaw).
Another newly cast actor is Vaughn regular Jason Flemyng, who will play the red, teleporting demon Azazel. It's probably not a huge part, but we can expect Flemyng will have a scene or two with Lawrence, since Azazel and Mystique are the parents of Nightcrawler. The actor did tell Empire that he'll appear with heavy make-up but that his tail will be done with effects in post-production.
What do you think of the added cast members? Are you worried X-Men: First Class will have too many characters?