Sporting geek-chic glasses and a tight hair-cut, Robert's character couldn't be more different from the moody gangland henchman Darren in RTE's Love/Hate.
The film also features Jonathan Rhys Meyers.
Sheehan is now being tipped to follow in the footsteps of Robert Pattinson (26) and become Hollywood's latest heart-throb. City Of Bones is the first chapter in the Mortal Instruments series.
Set in New York, the film is packed with fight scenes and bloodsuckers.
In the first instalment, lead character Clary discovers she is a descended from a line of "shadow hunters", a secret group of young half-angel warriors locked in an ancient battle to protect our world from demons.
Clary's ally is the super beautiful and super sarcastic Jace Wayland. Singer Phil Collins' daughter Lily plays the Clary while Rob plays her dorky boyfriend Simon.
Author of the Mortal Instrument series Cassandra Clare and Karate Kid director Harald Zwart said they knew Robert was just right for the role after watching him in Channel 4 series Misfits.
"I'm a big Misfits fan," Cassandra said. "So I'm ecstatic that our Simon is Robert Sheehan."