Stephanie Blacker tweeted this an hour ago:
And her agency has added the Game of Thrones mention back to their News. (It was deleted after WiC posted that they’d discovered her casting that way.)
Still no confirmation of who she is playing, but she’s filming in Belfast, so hopefully more info will leak. Thomas Brodie Sangster was also spotted in Belfast today. I think they’re still filming multiple storylines at once, and they’ve got stuff going on in Dubrovnik at the same time. Wild filming schedule this year.