The big question concerning Saoirse Ronan – apart from how on earth one pronounces her first name (try rhyming it with "inertia") – has been when exactly she might cross the threshold into adult roles, and how. The Irish actor has only just turned 19, so it's hardly surprising she has spent her career playing children, though that rather under-represents the challenging nature of her parts to date.

Unusually for a young actor, she has achieved fame and acclaim without first passing through the brightly lit high-school corridors of the teen-movie genre. She was Oscar-nominated at the age of 13 for her otherworldly performance as Briony Tallis, the spiteful catalyst for catastrophe in Atonement; she was the murdered girl monitoring the hunt for her own killer in supernatural drama The Lovely Bones; she played a ruthless assassin in Hanna; and in The Host, a teenage science-fiction thriller from the pen of Twilight creator Stephenie Meyer, Ronan was called upon to give a double performance: as both a blank-faced victim of extra-terrestrial possession, and the indefatigable soul still trapped inside.

Peter Bradshaw, Catherine Shoard and Henry Barnes review Byzantium guardian.co.uk

Given that she has technically never portrayed an adult on screen, Ronan is growing up in one colossal leap in her new film, Byzantium, an atmospheric chiller about a mother-daughter vampire team. Playing Eleanor the 200-year-old bloodsucker does dramatically raisethe average age of the roles on Ronan's CV, although she insists the secret was to approach her as the 16-year-old she appears to be on the outside. "I didn't really feel like I was playing an adult," she says in her gentle Irish lilt when we meet in a London hotel. She's wearing an emerald-green cardigan, and her fingernails are painted vivid electric-blue. Tissues and medication are spread out on the coffee table to help combat her cold. "Eleanor has been a teenager for 200 years," she points out. "But that's still what she is – a teenager. She's a very old soul. "

Some might say the same of Ronan. "She was the natural choice," says Neil Jordan, director of Byzantium. "I needed someone to play 16 or 17, and Saoirse has that gamine quality that can't be faked. But she also had this extraordinary stillness. When she begins to act, she is so totally possessed that it's quite spooky. People say that some actors are naturals. Well, with Saoirse, it almost feels unnatural."

You can see what he means: she looks positively haunted throughout most of Byzantium, not least when preparing to drain the arteries of her latest victim. Being a compassionate vampire, though, Eleanor selects her snacks only from the terminally ill, or from elderly people who express a readiness to die. But that doesn't keep the remorse from her face. What is it like, I wonder, to be on the receiving end of one of those extreme, penetrating closeups?

"A certain amount of technicality comes into it," she says. "It's about understanding the camera: how it works, what it will pick up. Sometimes when the camera is that close, it can be a bit, 'Oh – OK – er, hello.' But at the same time, you feel it's just you and the lens. It all goes very quiet on set. You can feel the atmosphere when everyone knows how important the shot is. The camera's like a friend sitting down that's just all ears and wants you to pour your heart out. It stares – that's its way of listening."

She may have played Eleanor as 16 rather than 200, but Byzantium still asks a new harshness of Ronan. "I do feel like the role is a step forward for me, maturity-wise," she says, "because of the graphic violence and the language I use. It's pretty stark. But I felt like I was ready." And she certainly has some challenging parts coming up: she will play the lead in Mary Queen of Scots, and has also been cast in Ryan Gosling's directorial debut, How to Catch a Monster.

When she isn't on set, Ronan lives with her parents in Dublin, having moved there a few years ago from County Carlow (though she was actually born in New York). Her father, Paul, is also an actor – he's had small roles in films including The Devil's Own and The Boxer – and now accompanies her during press duties (he nips in before our interview to discuss her outfit for a TV appearance later that evening).

If there's one striking thing about Ronan, it's her normality. She promises me, for instance, that she doesn't go in for vanity-Googling: "I really don't! I know lots of actors say that, but I honestly stopped doing it years ago. It's too distracting to read about yourself. You want to be perfect, and you want everyone to love you and that's never going to happen." And, although she'll talk about "Tilda", and refer in passing to "Ralph" as though he works at the Co-op, she is not above being endearingly starstruck herself. "Stevie Nicks came to a premiere I was at, and I nearly had a panic attack. And I couldn't believe I got to meet Paddy Smith." I confess I haven't heard of this Smith fellow, but it turns out her Irish consonants were merely rendering the name "Patti" more softly than usual.

During the shooting of Byzantium, Ronan was accompanied by her mother, who has always been present as a chaperone on her films. But more recently, while shooting Wes Anderson's The Grand Budapest Hotel this year (with Ralph Fiennes, Jude Law and Tilda Swinton), Ronan went it alone for the first time. "It was a bit strange at first," she confesses, "because me Mam has always been there on set, or made dinner for me afterwards. But we all felt it was the right time. I was worried about suddenly growing up and finding that people are still doing everything for me. It's not even just Mam – there's a lot of people on set who look after the actors. The whole thing is, 'You can't upset the actors!' I'm not into all that. I started feeling I needed to do more for myself." Such as? "I love cleaning. I love mopping the floor. If you need your floor mopped, I'm there."

• Byzantium is released this Friday. To enter a competition associated with the film visit Guardian Extra