The Northern Irish actor passed away on Friday, 8th Aug 2014, just four days after shooting his first scenes in the fifth series of the HBO show as Ser Denys Mallister, who was the oldest member of the show's military order, the Night's Watch.
According to the Belfast Telegraph newspaper, the star was due to shoot further scenes for the role and was said to have told pals he was excited to be working on the hugely popular TV series.
J.J's agent Philip Young paid tribute to the late actor following the news.
He said: "I have never encountered a man with more spirit, passion and love for his craft.”
As well as 'Game Of Thrones', J.J. was due to appear as a villager in upcoming movie 'Dracula Untold'.
He starred in several other movies throughout his career including 'Angela's Ashes', 'Mickybo and Me', and 'Cal'.
J.J., who is survived by his wife Mary, was also well known for his theatre work and played many parts at the Lyric Theatre in Belfast.
It is not yet known whether 'Game of Thrones' bosses will re-cast the role of Ser Denys Mallister.