The TROIKA casting website (rep. Michael Duff) have updated their database, and lo and behold: it seems we’ve found a gilly flower!
TROIKA’s site lists Hannah Murray's current project as playing the role of "Gilly" (named by her mother for the gilly flower), for the upcoming second season of Game of Thrones. Gilly is one of many young women living with old Craster (yet to be cast) — a crotchety and blunt "friend" of the Night's Watch. Gilly has a secret that threatens to sunder the already-uneasy peace between Craster and the northbound Night's Watch.
Murray is a young but well-regarded actress; her role as hapless Cassie on the British television series Skins caught many casting agents' attentions (it certainly caught mine), as Murray solidly played a quirky, loner character prone to fits of panic. Someone once described Cassie to me as "Luna Lovegood in a tizzy."
source 1, source 2
HBO has confirmed this casting to Westeros.org!