Established actress (2015–2021)
After starring in Stockholm, Pennsylvania (2015), a psychological thriller about Stockholm syndrome, Ronan played the lead role of Eilis Lacey, a young homesick Irishwoman in 1950s New York City, in the drama Brooklyn. Directed by John Crowley, the film is based on the novel of the same name by Colm Tóibín. Ronan believed that several aspects of her character's development and journey mirrored her own. The critical consensus on the review aggregator Rotten Tomatoes describes Ronan's performance as "outstanding". Peter Bradshaw of The Guardian considered it to be a "heartfelt and absorbing film" and wrote that Ronan's "calm poise anchors almost every scene and every shot". Kenneth Turan of Los Angeles Times took note of the "overwhelming empathy she creates with the subtlest means, the remarkable way she's able to create achingly personal, intensely emotional sequences while seeming not to be doing very much at all." Ronan received nominations for the Academy Award for Best Actress and the Golden Globe Award for Best Actress in a Drama.
In 2016, Ronan moved to New York City to begin rehearsals for her debut appearance on Broadway, in a revival of Arthur Miller's play The Crucible. She took the role of Abigail Williams, a manipulative maid responsible for the death of 20 people accused of witchcraft. Based on the Salem witch trials, the play was directed by Ivo van Hove and ran for 125 performances. In preparation, she read Stacy Schiff's book The Witches: Salem, 1692, and collaborated closely with van Hove to empathise with her villainous character. Instead of relying on previous portrayals of Williams, Ronan played her as "more victim than victimizer". Writing for The Hollywood Reporter, David Rooney considered Ronan to be "icy and commanding" and Linda Winer of Newsday commented that she had played the part "with the duplicity of a malevolent surfer-girl".
Ronan next voiced Marguerite Gachet in the biographical animated drama Loving Vincent (2017), and starred alongside Billy Howle as troubled newlyweds on their honeymoon in a film adaptation of Ian McEwan's novel On Chesil Beach. In a mixed review of the latter film, Kate Erbland of IndieWire thought Ronan was underutilised in it and that her performance had been overshadowed by that of Howle. She starred in Greta Gerwig's coming-of-age film Lady Bird, in which she played the titular role of Christine "Lady Bird" McPherson, a high school senior who shares a tumultuous relationship with her mother (played by Laurie Metcalf). It ranks among the best-reviewed films of all time on the review-aggregator site Rotten Tomatoes. Deeming Ronan's performance one of the best of the year, A. O. Scott of The New York Times wrote, "Ronan navigates each swerve in Lady Bird's story with an uncanny combination of self-confidence and discovery. She is as spontaneous and unpredictable as an actual 17-year-old ... which suggests an altogether stupefying level of craft." She won the Golden Globe Award for Best Actress in a Comedy or Musical; and received Academy Award, BAFTA and SAG nominations for Best Actress.
In 2017, Ronan hosted an episode of the NBC sketch comedy Saturday Night Live, in which one of her sketches was criticised for its stereotypical portrayal of Irish people, and featured in the music video for Ed Sheeran's song "Galway Girl". The following year, she starred in an adaptation of Anton Chekhov's play The Seagull, in which she played Nina, an aspiring actress. In a mixed review of the film, Michael O'Sullivan of The Washington Post praised Ronan's performance, writing that she "makes for an incandescent Nina, especially in her loopy final-act speech". She starred as Mary Stuart in the period drama Mary Queen of Scots, co-starring Margot Robbie as Elizabeth I of England. To maintain the distance between their characters, Ronan and Robbie did not interact with one another until filming their climactic encounter. Critic Todd McCarthy praised both actresses' performances and credited Ronan for "carr[ying] the film with fiercely individualistic spirit".
After becoming aware of a forthcoming film adaptation of Louisa May Alcott's novel Little Women, written and directed by Greta Gerwig, Ronan campaigned to play the lead role of Jo March, an aspiring author in the American Civil War era. In preparation, she read Marmee & Louisa, a biography about Alcott and her mother; the cast rehearsed the script for two weeks, and filming took place on location in Concord, Massachusetts. Little Women was released in 2019 to positive reviews. Richard Lawson of Vanity Fair took note of how well Ronan had portrayed her character "in all her conflicted loyalty, the struggle between her familial contentment and her yearning for something more". The film grossed over $218 million to emerge as her highest-grossing release. Once again, she received Oscar, BAFTA and Golden Globe nominations for Best Actress. This made Ronan—at 25 years and six months of age—the second youngest person to accrue four Oscar nominations, behind Jennifer Lawrence.
In 2020, Ronan portrayed the geologist Charlotte Murchison opposite Kate Winslet's Mary Anning in Francis Lee's Ammonite, a drama about a romantic relationship between the two women in the 1840s. The two actresses collaborated closely on the project, and they choreographed their own sex scenes. Steve Pond of TheWrap considered it to be "the most mature performance of [Ronan's] remarkable career". In the next year, Ronan had a small part in Wes Anderson's ensemble film The French Dispatch, about American journalists in France. She made her London stage debut at the Almeida Theatre, performing as Lady Macbeth in a revival of The Tragedy of Macbeth, opposite James McArdle. Ronan was intimidated by the experience of performing Shakespeare for the first time in her career, and drew inspiration from Kanye West and Kim Kardashian's marriage for portraying the relationship between Macbeth and Lady Macbeth. Alexandra Pollard of The Independent took note of Ronan's "rare skill to make Shakespeare’s beautiful but weighty words easy to understand".
