Saoirse Ronan on playing a vampire in Byzantium

“Vampire teeth really aren’t very efficient, are they?” she ponders. “It looks very messy. I’m not sure it’s the best way to get a pint off anyone.”

Well quite, so thank goodness Ronan plays a modern kind of vampire in Neil Jordan’s gothic Byzantium, where she and Gemma Arterton are mother and daughter, roaming Britain and digging their nails into veins. Nails? These vampires use an extendable thumbnail to pop veins like milk cartons. They also don’t have an allergy to crucifixes or daylight, though blood remains their drink of choice. “They don’t even call themselves vampires,” says Ronan. “The word is sucreants. It’s like they threw all the vampire traditions out of the window, and really, it’s a movie about loneliness because these two young women can’t have relationships or talk about what they are, because if they tell anyone, they have to kill them.”

It’s a far cry from the Twilight films. “It is, but I love vampire films. My favourite is Interview With The Vampire. It’s got this gothic sensibility, and, of course, Neil Jordan made that too, and the reason I made Byzantium is because I really wanted to work with Neil.”

At 19, Ronan seems a smart choice for a vampire teenager, since there’s something of an old soul about her. A keen follower of style, rather than fashion, she was thrilled by a recent Vogue photoshoot not because of the labels but because it meant she met the magazine’s grande dame, Grace Coddington – a name few other teens would throw around alongside Ronan’s other heroine, the more youth-friendly Lady Gaga.

Ronan is also precise about meaning, to the point of correctly punctuating her texts, and she has beautiful adult manners: at the end of the interview, instead of scuttling for cover, she stays on and chats easily until scooped up for a Q&A session for Byzantium with Arterton and Jordan. There, she proves a natural at handling crowd curiosity. There’s a risky moment where she offers a Scottish accent to an audience of Glaswegians, but she nails the cadence. “I did a film in Edinburgh called Death Defying Acts,” she says, by way of explanation.

This is Ronan at her most cool, calm and inflected, but onscreen she packs a wallop. Aged 13, she got James McAvoy accused of rape in Atonement, then haunted Stanley Tucci from beyond the grave in The Lovely Bones. At 16 she was a genetically modified child assassin in Hanna. “I like my freaks,” she says. But she doesn’t always enjoy what goes along with them. For her last film, The Host, where she plays a human possessed by an alien, she had to wear special contact lenses. “God that hurt,” she says. “You can only see a little bit through the centre, like tunnel vision, and it was so painful. Sometimes in the film, where I’m looking upset, that’s the lenses.”

The Host was also her first brush with a new kind of film fame. Until then, she’d been a poster child for the arthouse circuit, fascinating to watch, and Oscar-nominated for her preternatural performance in Atonement. In person it’s not her looks but her ease that is startling. It’s not a hot-housed precocity, but a natural, cheerful confidence that only falters when we talk about the possibility of another film, bringing potentially rabid, Twilight-level fame to her doorstep, especially since both The Host and the vampire saga are based on books by Stephenie Meyer.

“It’s a different kind of attention, a lot more intense and a younger audience. They’re more involved in social media. They tweet and use Facebook. It’s much more immediate, and when you see the reaction at premieres to Twilight, or the Harry Potter people, it looks a little extreme.” Then she adds: “But I don’t really think The Host is that kind of film.” It’s as though she is trying to reassure herself, but the film, released last month, makes her meaning clearer: Ronan always knew The Host, although popular with tweens, was not destined to do Twilight style-box office.

In any case, she’s familiar with scrutiny in Ireland, where Ronanmania is rife. It helps that she is from acting stock: her father Paul is a veteran of TV and films, and introduced his daughter to the camera as a baby while filming Veronica Guerin with Cate Blanchett. She made more of an impression aged nine in the RTE soap The Clinic. Casting directors noted a new child actor with a facility for emotional depth and marked her card. At home, there’s great interest in whether she has a boyfriend (she certainly has a best friend called Chris, who is a boy), and enormous, justified pride in a Carlow girl whose other boy pals “Peter” and “Ryan” are Lord Of The Rings director Peter Jackson and current pinup Ryan Gosling. Does she ever find this A-list coverage wearing? “Well I know most of the photographers in Ireland,” she says. “And if I don’t want my photograph taken, they will leave me alone.”

Byzantium has reignited talk of boyfriends and romance, because her character is drawn to a frail teen, played by Caleb Landry Jones. However, as Ronan briskly points out, she’s been kissing boys for the camera since she was ten. “As long as you feel comfortable, it’s just another day,” she says, although Max Irons in The Host caused her some soul searching. “He smokes like five cigarettes between takes,” she rolls her eyes. “And he drinks coffee. My make-up artist looked after me though. If she saw him puffing away, she’d be after him with mints, saying ‘You need to have this before you go near my baby.’”

Irons is the son of Jeremy, and despite the fag breaks, they bonded over acting dynasties, and an Irish upbringing: his father owns a stately pile over there. “Max understood the Irish way. We could tease each other and he’d take it for what it was.” At the Twilight premiere they attended, someone kept calling him Jeremy “which I thought was really funny. So I’ve been calling him Jeremy since”.

Home-schooled since her teens, Ronan says she would like to go to university, although it’s hard to see when she will get the chance since she has filming booked for the next two years.

She would also like to follow her Atonement co-star McAvoy into theatre. “Have you seen him in Macbeth?” she asks enthusiastically. “He’s great. I’d love to do something like that, although perhaps not quite so physical. He’s got the bruises to prove it already.”

Ronan’s own immersion in Scottish accents, it turns out, is for an upcoming period drama about Mary, Queen of Scots, directed by the Oscar-winning Danish filmmaker Susanne Bier. “I’m still reading up on the history, but it’s really fascinating. There are a lot of similarities between Scottish and Irish history, so I really hope this happens.”

First, however, she has a role in How To Catch A Monster, written and directed by Gosling. “I can’t say much about the character because he asked me not to, but we had emailed back and forth and then we sat down and talked about his film. He’s very clear about what he wants. And he’s an actor himself, so he gets it.” «

Twitter: @SiobhanSynnot

Byzantium is in cinemas from 31 May