It looks like actor Ricky Champ will be joining the cast of Game of Thrones for season 6. According to the actors CV, Champ will be taking on the role of Flynn.
Thanks to a previous casting list we have a brief description of the character of Flynn: "He's part of an outlaw band. The group is using religion to justify terrorizing and extorting what they need from the people of the countryside." The character will be appearing in two episodes in season 6.