Especially known for her acute and sensitive portrayals of young women in period dramas, as well as her ability to portray the complex characters of more modern tales, Saoirse Una Ronan was born in Bronx, New York City on 12 February 1994 to Irish parents Paul Ronan, a construction worker and a bartender, before becoming a stage actor, and Monica Ronan, who worked as a nanny and had acted as a child. Interestingly, in Irish language, her first name Saoirse means Freedom and is pronounced as "Seer-sha" in Ireland, and her middle name Una means Unity, while her surname Ronan is also Irish and means Little Seal. Nevertheless, when Saoirse was only three, her undocumented immigrant parents moved back to their native Dublin and settled in the small village of Ardattin in County Carlow, located in the Southern Region of Ireland, where Saoirse initially attended Ardattin National School for a time, before being tutored at home.

Saoirse made her screen debut as a child in 2003, when she appeared in a few episodes of the medical TV series The Clinic, and then in four episodes in the crime series Proof (2005). Immediately after that, although little Saoirse was not successful in her audition to grab a role in the fantasy film Harry Potter and the Order of the Phoenix in 2005, she made her film debut in the same year in Amy Heckerling’s romantic comedy film I Could Never Be Your Woman, which was released much later in 2007. In the same year, she also got her breakthrough role of Briony Tallis, a 13-year-old aspiring novelist, whose false accusation against her sister's lover, driven by her immaturity and a misunderstanding, shattered their course lives forever, in the period drama Atonement (2007), directed by Joe Wright. For her performance in the role, Saoirse was profusely acclaimed by the critics and the commoners and was nominated for an Academy Award for Best Supporting Actress, creating history to become the seventh-youngest nominee in the category, in addition to earning nominations for a BAFTA and a Golden Globe Award. The same year, Saoirse Ronan also earned praise for her performance in the psychological thriller Death Defying Acts (2007), in which she played the role of Benji McGarvie, the daughter of an impoverished and uneducated con artist performing psychic acts, played by Catherine Zeta-Zones.

The next year Saoirse appeared as Lina Mayfleet in the sci-fi children’s adventure film City of Ember (2008), which received a mixed critical reception and failed at the box office. However, the next year she starred in The Lovely Bones (2009), a supernatural drama film, directed by Peter Jackson and co-starring Susan Sarandon, in which she played the role of a raped and murdered 14-year-old Susie Salmon, who watches her family and her killer from the afterlife and comes to terms with her quest for vengeance. The film was a box office disappointment, but it earned Saoirse Ronan a BAFTA nomination for Best Leading Actress. She continued her success in the next year also, when she co-starred Colin Farrell and Ed Harris, playing the supporting part of Irena, a Polish orphan during World War II, in the war drama The Way Back (2010), directed by Peter Weir, and won Irish Film & Television Award for Actress in a Supporting Role.

The following year, Saoirse Ronan reunited with Joe Wright, the director of Atonement, to play the title character in the action thriller Hanna (2011), co-starring Cate Blanchett, about a 15-year-old girl raised in the Arctic wilderness and trained by her father, as a self-sufficient assassin. To make herself perfectly prepared for the role and perform her own stunts in the film, Saoirse spent several months of hard training in martial arts, stick fighting and knife fighting. Hanna was proved to be a moderate box-office success, but the performance of Saoirse, as well as her action sequences, was highly praised by critics. After that, Saoirse appeared in the action film Violet & Daisy (2011), directed by Geoffrey Fletcher in his directorial debut, followed by Neil Jordan’s dark horror vampire drama film Byzantium (2012), which gave her an opportunity to portray a completely different type of role, the role of Eleanor, a young vampire. Her films in the next year include Andrew Niccol’s science fiction thriller The Host (2013), in which she played the dual role of Melanie Stryder, which is a human rebel, and Wanderer, a harmful alien. In the same year, she also played the role of Daisy, an isolated American teenager in a distant country during the outbreak of a fictional World War III, in the romantic speculative drama film How I Live Now (2013), and was nominated for a BAFTA and a Saturn Award for Best Actress. The next year, Saoirse Ronan played a mysterious young girl named Rat in the surrealistic fantasy film Lost River (2014), and also appeared as Agatha, an apprentice baker, in Wes Anderson’s highly acclaimed ensemble comedy film The Grand Budapest Hotel (2014), which was a box office hit, and was nominated for nine Academy Awards.

The next year, apart from playing the role of Leanne Dargon in the psychological thriller Stockholm, Pennsylvania (2015), Ronan also played the lead role of Eilis Lacey, a young homesick Irishwoman in 1950s New York City, in the romantic period drama film Brooklyn (2015), which added a new feather to her crown. Her evocative and absorbing performance in the film was highly appreciated by the critics, and she earned an Academy Award nomination for Best Performance by an Actress in a Leading Role, becoming the second-youngest actress to receive two Oscar nominations at the age of 21. Her next important film is Greta Gerwig’s coming-of-age film Lady Bird (2017), in which she played the titular role of Christine "Lady Bird" McPherson, a high school senior who shares a tumultuous relationship with her mother. For her spontaneous performance in the film, portraying the character of a teen girl with an uncanny combination of self-confidence, she earned profuse applause from the critics and was depicted by the newspapers as one of the most formidable screen actors of the day. For her stunning performance in the film, she was nominated for an Academy Award for Best Actress, making her the second-youngest actress (after Jennifer Lawrence) to receive three Academy nominations before the age of 24, along with nominations for BAFTA and SAG Awards, in addition to earning the Golden Globe Award for Best Actress in a Motion Picture (in a comedy or musical).

More praise followed Saoirse Ronan the following year for her performance as Nina, an aspiring actress, in an adaption of Anton Chekhov’s s play The Seagull (2018), and in the period drama Mary Queen of Scots (2018), playing the ill-fated title role of Mary Stuart. After that, she portrayed the strong-minded tomboyish Jo March in Gerwig’s highly praised and commercially successful coming-of-age drama film Little Women (2019), for which she again received BAFTA, Golden Globe, along with her fourth Academy Award nomination for Best Actress, becoming the second-youngest person to accrue four Oscar nominations, behind Jennifer Lawrence. Next year, Ronan starred in Ammonite (2020), directed by Francis Lee, portraying the British geologist Charlotte Murchison opposite Kate Winslet, playing the role of Mary Anning, an English palaeontologist, fossil collector, and dealer. In the film, a drama about a romantic relationship between the two women in the 1840s, rated R for graphic sexuality and some graphic nudity, the two great actresses belonging to different age groups collaborated closely on the project, and they choreographed their intimate sex scenes.

The subsequent films of Saoirse Ronan include The French Dispatch (2021), in which she reunited with Anderson to play the minor role of Junkie, the comedy mystery film See How They Run (2022), playing constable Stalker solving a murder in 1950s London, and Garth Davis’ intimate science fiction thriller, Foe (2023), followed by The Outrun (2024), a drama film directed by Nora Fingscheidt, about Rona, a recovering alcoholic who moves back home, for which she earned a BAFTA Award nomination for Best Leading Actress.

Noted for playing complicated female characters in carefully selected projects and depicted by several critics as unpretentious, but one of the finest actors of her generation, Saoirse is known for guarding her private life. However, it is reported that in her personal life, she had been in a relationship with Scottish actor Jack Lowden, her co-star in Mary Queen of Scots, and they had married in a secret ceremony in Edinburgh on 20 July 2024. It is reported that the couple is expecting their first child soon.