Saoirse Ronan has beautiful pale-blue eyes. Every director she has worked with has chosen to focus on this at some point, because they express so much. As Ian McEwan said of her breakthrough role in the film of his novel Atonement, 'She gives us thought processes right on screen, even before she speaks, and conveys so much with her eyes.’ Which makes it all the more distressing when, during our meeting, they suddenly fill with tears.

I am telling her how much I enjoyed her latest film, Brooklyn, which went to Sundance Film Festival early this year as a small indie vying for attention and came out as an Oscar contender. Ronan ends up apologising for getting emotional. 'I’ve never worked as hard as that, and I definitely needed a bit of emotional support because it’s too close to home,’ she says.

'For people to respond to it as well as they have – I have to say it’s a dream.’ She has not seen the film, she admits later. 'I can’t. Just talking about it, you can see I’m a basket case. In a couple of years, or when I have kids or something, we’ll all sit and watch it together.’

Adapted by Nick Hornby from Colm Tóibín’s novel, and directed by John Crowley, Brooklyn tells the story of Eilis Lacey, a shy Irish teenager who is forced to leave her family to look for opportunity in New York in the 1950s, and eventually blossoms there.

Saoirse Ronan in her new film, Brooklyn. Credit: Courtesy of Lionsgate

It portrays a side of the Irish diaspora rarely seen on film: the wrench of leaving home, the culture shock of New York, the community of women in particular that helps Eilis settle into her new life, and ultimately a choice between two different but equally valid ways of life.

Ronan shines as Eilis, convincingly growing from girl to woman and – although she was only 20 when the film was made – carrying off her first truly adult role with style. It is hard to imagine her ever making a film more personal than this one.

Her own parents, Paul and Monica, left Ireland for New York during the recession of the 1980s, in much the same circumstances. Ronan, their only child, was born in New York in 1994, although she has few memories of living there – her father’s work as an actor brought the family back to Ireland when she was three.

With Juno Temple in Atonment aged just 12, a performance for which she was nominated for an Oscar. Credit: Rex Features

'I only realised in retrospect that it is essentially their story,’ she says. 'They had nothing, and they were illegal for a few years so they couldn’t leave because they wouldn’t be allowed back in. My dad started out in construction, then he became a bartender, and eventually he was discovered by another Irish actor in the bar.

My mum was a nanny, and she was adamant that she was going to have me there so I wouldn’t ever have to go through the tough time that they had in order to get their visas.’ Ronan has dual US/Irish citizenship. 'I’m blessed in that I can just move to New York or LA if I want.

Even when it comes to work, I don’t have to worry about visas. So thanks, Mum! She came to see the film at Sundance, and for me she was the most important critic, because I wanted her to feel like it had authentically portrayed her struggle.’

Starring opposite Paul Rudd and Michelle Pfeiffer in her first film, I could Never Be Your Woman. Credit: Rex Features

What makes the film even more personal is that the Irish scenes were filmed in Enniscorthy. 'I grew up 20 minutes away and used to go there to the cinema,’ Ronan says. This led to some surreal moments – such as filming a dance-hall scene and realising that many of the extras were people she went to school with, all dressed up in 1950s costume.

It was hard, she admits, immersing herself in a character while mixing with people she had played basketball with at school. 'That world, that part of me, wasn’t a part of this other world, so for the two to mix like that was weird.

But it was so nice to be able to shoot on the street where Colm was from, and to have the people of Enniscorthy be a part of it. This film is theirs – we’re going to have a screening there for them. It was exciting for all of us to share that, but it was also like staring at a mirror, and you couldn’t look away from it or hide from it.

It was like an open wound, and it was really exposing. I was convinced that I was doing a terrible job; I’d call my mum up, especially those first couple of weeks, convinced that I was going to let everyone down.’

Ronan in The Lovely Bones (2009) Credit: Rex Features

If all this appears a little precious in print, it does not feel that way in person. Wearing casual clothes and no make-up, Ronan is refreshingly down-to-earth for someone who has spent most of the last decade on film sets.

Her childhood, she says, was far from theatre brat, although she was sometimes called in if her father was in something where a baby or toddler was required.

Her first brush with Hollywood was meeting Brad Pitt at the age of two: her father was working with him on the IRA drama The Devil’s Own. 'I’m sure Brad remembers it really well,’ she says drily when I ask about this. 'It’s one of the anecdotes he’ll bring out at a party.’

I just like pretending, and I like making that seem real. If you can hold on to that kind of childlike fascination with the imaginary world when you’re older, it’s the best Saoirse Ronan

At home, she liked putting on shows. 'I was convinced that I was going to be Jean Butler from Riverdance when I grew up.’ When she was seven, she did a few hours’ work on a film, although she was not keen at first. 'They needed a kid to dress up as a clown for this weird, arty flashback. And I loved being on a film set. I think Dad kind of saw that in me, and he mentioned it to his agent.’

Her proper acting debut was in an Irish television show called The Clinic, in 2003. 'I was so nervous beforehand, but as soon as I got on set I felt completely at home. I’ve always been a bit of a perfectionist, and I liked the challenge of somebody asking you to do something and then trying to do it.

And it was the atmosphere of being on a set: there are so many people there. There was a scene where the fire alarm goes off and everything’s a bit manic, and I loved watching the other actors pretend to be scared and stressed, and following suit.’

She was 11 when she made her first film, I Could Never Be Your Woman, in which she played Michelle Pfeiffer’s daughter with an impeccable American accent.

She worked with a dialect coach, who later mentioned her to Atonement director Joe Wright, leading her to play Briony Tallis, an imaginative 13-year-old whose incomplete understanding of events one summer’s day destroys lives.

Wright wanted a specific walk for Briony, and a particular voice, and even now Ronan says the walk and the talk are her first ways into a character. 'Even for a 12-year-old, it was very thought-provoking. There was so much emotion that you had to think about and calculate.'

At the Grand Budapest Hotel premiere in New York. Credit: Getty Images

'We had the best crew and the best cast, and I was really looked after by the likes of James [McAvoy], Benedict [Cumberbatch] and Keira [Knightley]. I was in heaven. That’s when I knew I loved being an actor. And that hasn’t gone away yet. I just like pretending, and I like making that seem real. If you can hold on to that kind of childlike fascination with the imaginary world when you’re older, it’s the best.’

Ronan was nominated for the best-supporting-actress Oscar in 2008 for Atonement, becoming one of the youngest actresses ever to be honoured by the Academy. She was working in New Zealand, filming The Lovely Bones with Peter Jackson, so she and her parents flew to LA for the ceremony and left within 24 hours. 'I was in a bit of a daze,’ she says. 'It was all mad and went over my head a little bit.’

Monica met her hero, John Travolta, the highlight of the night. Ronan did not expect to win, so when Tilda Swinton’s name was announced, all she felt was relief that she did not have to make a speech.

When you’re playing a character, you do what you have to do. If you’re supposed to be a psychopath, you’ll become a psychopath Saoirse Ronan

'More than anything, it was exciting to be part of a show that I had seen on the telly. If I was to meet Dermot O’Leary, Ant and Dec or someone from Orange Is the New Black, it would excite me more than meeting Jack Nicholson. And maybe music – if I met Stevie Nicks, I don’t know what I’d do!’

Since then, Ronan has moved smoothly from film to film. Her mother always accompanied her. 'I was never exposed to anything that would have exploited me as a kid. I didn’t go to all those parties, but I wasn’t kept from it either, so I grew up [around] a lot of different characters and egos, and had just enough of a taste of it without it sucking me in.’

In 2012 she moved to London for a while, alone. She was at an age when many people go to university – an experience she says she may still like to try, at some point – and it was time to assert her independence a little. 'Mum and I went everywhere together, and then a lot of the time Dad started to come away with us as well. We were a very tight unit, and I was ready to break away from that a bit.’

'It’s become a lot more important to me to help progress women in film' says Ronan Credit: Mary McCartney

Her parents were supportive, she says, but it was still a wrench for all of them, and she feels that is why Brooklyn has such wide appeal: most of us have to leave home at some point, or need to let our children make their way without us.

The first film she made without a chaperone was Wes Anderson’s The Grand Budapest Hotel in 2013. Her mother still went to Germany for the first couple of days, Ronan says fondly, to settle her in. 'She made sure I had my grub and my hot-water bottle.’

Working without a chaperone has given her a fuller experience, she adds. 'When your parents are there, especially when you’re younger and you’ve got tutoring to do, you’re removed from the rest of the group somewhat once the day is over. Now it’s become much more of a social thing, and the friends I’ve made are really great pals who I meet up with often. It’s definitely changed the relationships I have with people; it’s made that stronger.’

We meet in early August, just after Ronan has finished a 21-day shoot for an adaptation of the Chekhov play The Seagull. Next she is going to stay with her parents prior to moving to America for a while. 'I’m going to be doing something in New York and I can’t talk about it yet – which is crap, because I really want to.’

Shortly afterwards, it is announced that Ivo van Hove will be directing Arthur Miller’s The Crucible on Broadway in February. There will be an original score by Philip Glass, and Ronan will star alongside Ben Whishaw, Ciarán Hinds and Sophie Okonedo – not a bad way to make your theatre debut.

Ideally, she tells me, she would divide her time between Dublin and New York, visiting friends in London. 'But I’ve learnt not to make a plan. You can get one phone call and it changes everything.’

She is not often offered the kind of roles that used to be standard for young women – characters that exist to be rescued by the hero, or as a prop for his virility – although she was once asked to read for a part to prove that she could do sexy.

Saoirse Ronan in Ryan Gosling's 'Lost River'

'I thought that was funny. When you’re playing a character, you do what you have to do. If you’re supposed to be a psychopath, you’ll become a psychopath; if you’re supposed to be a sexy character, if you’re a good actor and you can get into that place, you’ll be sexy. It doesn’t mean that you are like that in real life, or that you’re not. I don’t necessarily think about myself in that way, but maybe that’s because I’m younger.’

Things have changed a lot for women in film over the last decade. I Could Never Be Your Woman was denied a theatrical release in many territories, going straight to DVD in America in 2008 because distributors doubted that there was a market for a film about a 40-year-old single mother and her daughter, or that an older actress could attract an audience.

It is something Ronan has been thinking about. 'It’s become a lot more important to me to help progress women in film, whether it’s through the characters that we see on screen, or just by working with female filmmakers so that it becomes the norm for a woman to be a director, not a talking point.’

What excited her about Brooklyn, she adds, was that the female characters 'aren’t glamorised or overly sexualised – they’re normal women. This was an intelligently written character who really went through a journey, and there were so many scenes where she only interacted with other women.’

With Domhnall Gleeson in Brooklyn where she plays an Irish girl in New York. With Domhnall Gleeson in Brooklyn where she plays an Irish girl in New York. Credit: Courtesy of Lionsgate

Having taken on a more adult role, she says, 'I’d find it quite odd to go back and play someone who’s 16, 17 and maybe hasn’t lost their virginity yet. I’m not ready to go back to that really innocent stage yet. But I never really did children’s films. I was always in quite a mature environment when it came to the story. I was so ready to do something like this, but it’s tricky. You’re 19, 20 and the last time producers or directors saw you in something, you were 15.

'I felt I was hovering between two age groups for a while, and I couldn’t quite land in either, and then Brooklyn came along and it was perfect. When I was doing the film last year, it was an emotional shift for me. And I felt there was a lot in my own life that was shifting too. It’s such an interesting time for women, and I don’t know why it’s not written about more.

'There’s so much that’s changing for you at this age, and so many adjustments that a woman has to make when it comes to her friendships and relationships, work and sex, and how she feels about herself and where she stands on certain things. I’m going through all of that now, so I feel it’s important to see a woman going through that on screen.’

Brooklyn is out on November 6.