The role is that of Madeline Madrigal, aka Ma-Ma. In the Dredd script she's described as a woman in her late fifties or early sixties with a big scar through her cheek and lip. I can imagine they'll just ignore the first part of that description, and the only make up Ms. Headey will be subject to will provide the scar tissue but not age her.
Ma-Ma is the leader of a gang who over run the Peach Tree City Block. Almost all of the film takes place here, an extended action sequence not too dissimilar, I guess, to the Die Hard paradigm.
The casting has not been officially announced yet but I'd heard about it, and then it turned up on a Dredd fansite with a rock-solid track record, so I'm going to roll with it. They say she started work on the picture last week.
Don't forget our previous posts filled with images from the film's set. This is not your grandfather's Judge Dredd. No. Especially not if your grandfather is Danny Cannon or Sylvester Stallone.