In the RTE series, Tommy's girlfriend Debbie was subjected to a horrifying attack by Real IRA leader Git -- which was one of the most brutal scenes ever depicted on Irish TV.
"Watching it and filming it were very different," Killian (27) told the Herald.
"Filming was very functional and mechanical. It was in a cramped environment and we were all day shooting the scene. It is so explosive.
"I had to do a lot of violent physical activity -- but you just have to make sure no one gets hurt.
"It is such volatile, high-energy scene and everyone was focused on what you had to do.
"Watching it was very different."
He explained: "I watched it with a few friends the night it aired -- I found it quite brutal to watch.
"My friends who are 27/28-year-old young men -- their response was that it was not just shocking for RTE, but shocking anyway. It was so graphic and brutal."
But Killian said that this type of scene needed to be shown in this way.
"Personally I do think it was necessary," he said.
"It is something in life that is a very brutal and horrific thing.
"The manner in which it was depicted was necessary for the kind of world it was set in.
"I thought it was done well, it done right and it was very powerful. It is the single incident that catapults the entire series."
Killian's character Tommy is about to come into his own and improve his standing in the gang in coming episodes.
But he said there is more drama on the horizon for the troubled Dublin character.
Killian landed his first serious acting job with the hit RTE show and since then his career is on an upward trajectory.
He has just wrapped a movie called Calvary, starring alongside Brendan Gleeson, Chris O'Dowd and Dylan Moran, and directed by John Michael McDonagh, who was behind the acclaimed The Guard.
And he has his sights set firmly on the US once the Dublin-based crime drama wraps.
But for now he is relishing the gradual development of the character of Tommy in the acclaimed series.
Tommy has been a blindly loyal lieutenant to gang leader Nidge.
Killian said that this series we will see a little more development to the character as he becomes even more central to the storyline.
Tommy is no angel -- he is cheating on his girlfriend and mother of his child Siobhan (Charlie Murphy) with drug addict prostitute Debbie (Susan Loughnane).
And last week saw him flirt with danger as he sparked up a fling with RIRA boss Dano's girlfriend Georgina (Eve Macklin).