Saoirse had talked about playing Scarlet Witch not very long ago, as you can read here, and she has now addressed the rumors once again.
“[Joining Avengers 2 as Scarlet Witch is] not happening, no! I think that was just one of those little rumours floating about.”
Considering the casting for Wanda (Scarlet Witch) has already begun, it is possible that Saoirse is actually not in the running. Unless, of course, she’s been told to keep her involvement secret, although I’m not sure why that would be necessary. Maybe, if her casting is part of a big Comic Con reveal, or other such giant media event. Perhaps we shall find out soon?
Source: The Mary Sue