The image comes from Neil Jordan's eagerly awaited movie Byzantium, which is due out later this year.
Saoirse said she took the role of a teenage bloodsucker as she liked "the idea of playing a teenager who was 200 years old and was sort of stuck in limbo".
The Carlow girl said working with directing legend Jordan was an incredible experience.
"I've wanted to work with him for a while. He is so humble and lets actors do their own thing," she said.
In the clip, which is not for the faint of heart, Saoirse watches as an injured young man hobbles into a car.
As he shuffles past, he drops a blood-sodden rag to the ground, which Saoirse then picks up and stuffs in her mouth, drinking the young man's blood.
Saoirse (18) plays alongside Clash of the Titans star Gemma Arterton in the film, which follows two mysterious young women who flee the scene of a crime and end up in a remote, run-down coastal town.
But they are not run-of-the-mill vampires.
"They don't have fangs or anything," Saoirse explained.
"Instead they have a talon. When they're hungry and are about to feed, the talon gets longer."
Partly shot in Bray, Co Wicklow, Byzantium premiered at the Toronto International Film Festival earlier this summer, but as yet has no release date for Ireland.
This is not the first time that Neil Jordan has delved into the gory genre: he directed the acclaimed Interview With The Vampire starring Brad Pitt, Tom Cruise and Kirsten Dunst.
Saoirse was thrust into the limelight after being nominated for an Oscar at the age of 13 for her performance in Atonement, alongside Keira Knightley.
Since then, she has worked in a number of critically acclaimed films, including The Way Back and The Lovely Bones.
But it looks as if she's sticking with the supernatural theme.
The blonde beauty is set to play the lead role of Melanie in Stephanie Meyer's upcoming film, The Host. Meyer is the woman behind the hugely successful Twilight series ... which is all about vampires, werewolves and unrequited love.