User blog comment:Merlin artist/happy birthday Katie Mcgrath!/@comment-5822565-20121027105326/@comment-5209013-20121027152603

Apparently it was a rumour that has been around for years. On October 25, she and Colin did a radio interview in which she said that October 24 isn't her birthday.

Right now on Tumblr they're saying that it's February 21, apparently someone has asked her at Comic Con :P