It's rare to watch a film and realize it’s becoming a classic in front of your eyes, but that’s the case with "Brooklyn," the timelessly beautiful new Irish emigrant story starring Saoirse Ronan.

Set in the early 1950s, Ronan plays Eilis Lacey, a young Enniscorthy girl whose emigrant journey to Brooklyn changes her life in ways that are far from comfortable for her, at least at first.

Based on the award-winning novel by Colm Tóibín, the author has described the story as “quite low key, about somebody very ordinary.” That’s true, but it’s actually the film’s everyday reality that gives it its unique power.

Generations of the Irish have been lured to these shores by the promise of a new start, but there are harsher realities to contend with too, as Eilis discovers when she’s filled with a terrible homesickness in the first few weeks of her new in New York.

Every Irish person who has ever taken an emigrant boat or plane to a new life abroad will know Eilis’ journey in their bones, and "Brooklyn," is masterful in the telling of it. But in a storyline that’s so rich with sentiment, the film is never sentimental.

In other hands, director John Crowley’s "Brooklyn" could have become a by-the-numbers period piece replete with Irish eccentrics making distracting cameos. Instead, he turns "Brooklyn" into that rare thing, an unforgettable portrait of the Irish in full, with all their contradictions and capacities for good and ill.

Eilis’ pining for home is quickly offset by the unexpected romance that blossoms with Tony Fiorello (Emory Cohen) a young Italian American trainee plumber she meets at a dance in the parish hall.

The chemistry between Ronan and Cohen, the two young leads, is so strong that it lights up the film. Cohen’s character looks utterly bewitched by Ronan’s, and the scenes where he woos her and introduces her to his Italian American family are among the funniest and most beguiling in the film.

Crowley paces the film expertly and his camera work is both low-key and instantly iconic. He cares deeply about these characters, and in this story and you can see it in every frame.

So "Brooklyn" is the kind of film where film-goers will argue over which scene is the most memorable (in a film filled with them) and Ronan and Cohen are consistently superb together.

Eilis’ uncertainty about which life to commit to – her old one in Ireland or her new one in America – becomes an even tougher question when, after a death in the family, she returns to Ireland for a time.

Back home in Enniscorthy, she runs into Jim Farrell (Domhnall Gleeson) a local rugby club lad from a prosperous family with a very bright future ahead of him.

Jim represents a road not taken, but his timing isn’t ideal. Had Eilis met him before she left for America, and had she landed the job she’s been offered on her return, she might never have moved to Brooklyn in the first place, she realizes.

Jim’s appearance deeply complicates the picture for Eilis, who is forced to choose between two goods, two men and two nations. That her decision is a far from foregone conclusion is down to the skill of the directing and performances, and "Brooklyn" will keep you guessing all the way to the rapturous final frame.

The work of the supporting cast is uniformly excellent. Julie Walters plays Miss Kehoe, Eilis’ Irish boarding house mother, with a deep understanding of the attitudes of the period, and Jim Broadbent does terrific work as the benevolent Irish parish priest who sets Eilis up with a job and a place to live when she reaches Brooklyn, then guards over her.

But it’s Cohen and Gleeson who coax Ronan’s shy character out of her shell and guide her toward love and life. Along the way, she also has some searing run-ins with the kind of Irish people you won’t see in the tourist brochures. As Miss (Nettles) Kelly, Brid Brennan is a reminder of the kind of sanctimonious busybodies that every little Irish town has a surfeit of.

"Brooklyn" paints a remarkable portrait of the challenges and rewards of emigration and it cuts no corners in the process. We see the hardships as well as the happiness.

Along the way, Eilis becomes an Irish Everywoman and her tale becomes every Irish emigrant’s tale. And by making such an Irish film Crowley has in the process crafted the perfect American one too.