The Fringe actress will appear opposite Matt Smith in the actor's final Who episode.
DS can exclusively reveal that Brady's character is someone from the Doctor's past, with the plot of the festive episode revolving around her.
The Irish star's past credits include BBC One's Mistresses, ITV's Eternal Law, Sky1's Sinbad and US sci-fi drama Fringe.
Smith's final outing as the 11th Doctor is also expected to feature classic villains The Cybermen.
Jenna Coleman - who plays companion Clara - recently described her co-star's departure as "brutal" and "traumatic" to film.
Peter Capaldi has been cast as Smith's successor and will star in his first full series of Doctor Who in 2014.
> Jenna Coleman on Matt Smith's Doctor Who exit: "It's perfect"
Watch a clip from Doctor Who's 50th special below: