Farrell was born prematurely, weighing 1 pound 6 ounces, in Dublin to Rita and Eamon Farrell. His father was a footballer who played for Shamrock Rovers FC. Farrell has three siblings, two sisters, Claudine and Catherine, and a brother, Eamon Jr.
Farrell was educated at St. Brigid's National School in Castleknock followed by Castleknock College and Gormanston College. In the late 1980s and early 1990s, he was a promising young football player for Dublin team, Castleknock Celtic, as a goalkeeper. Farrell attended The Gaiety School of Acting, but dropped out and was cast in the part of Danny Byrne on Ballykissangel, a BBC television drama. Farrell appeared on the show from 1998 to 1999.
Farrell had a number of small parts in various TV shows and movies, until 2000, when he was cast in the part of Private Roland Bozz in Tigerland, an American film directed by Joel Schumacher. Farrell's next American films, American Outlaws (2001) and Hart's War (2002), were not commercially successful, but his 2003 films, including Phone Booth, S.W.A.T. and The Recruit were well-received box office successes. Although he has a pronounced Irish accent, Farrell uses an American accent in some of his films including American Outlaws and his breakthrough role, Tigerland. Farrell is also a proven supporting actor, given his performances as an ambitious cop who chases after a potential criminal, played by actor Tom Cruise in Minority Report (2002), and as the skilled villain Bullseye in Daredevil (2003).
Farrell as Officer Jim Street in S.W.A.T. (2003).In 2004, Farrell appeared in several films that received only a limited theatrical release, including Intermission and A Home at the End of the World, both of which received some positive reviews.
Farrell appeared in the title role of Alexander the Great in 2004's Alexander, which received mixed reviews. The film was marked by controversy, including homoerotic scenes with other men. Both the movie, including Oliver Stone's directing and story telling of Alexander, and Hephaistion's bisexual aspects, were heavily criticized by some historians and film critics who saw it as an inaccurate and misleading representation of the characters' sexualities. Though it failed at the US box office, it made a profit on its overseas revenues.
Farrell also appeared as a bisexual character in A Home at the End of the World. He has credited his homosexual brother, Eamon Farrell Jr., for inspiring him and teaching him about "the meaning of love and understanding."
Farrell as Alexander the Great in Alexander (2004).His next film was 2005's Academy Award-nominated The New World, also a historical epic that was met with mixed reviews. Farrell played the leading role of captain John Smith, the founder of 17th century colonial Jamestown, Virginia who falls in love with a beautiful American Indian maiden, Pocahontas, played by actress Q'Orianka Kilcher. The film achieved success, despite being released in 811 theatres worldwide and having a relatively low box office gross.
The New World was followed by Ask the Dust, a romance film set in period Los Angeles co-starring Salma Hayek. It received a very limited theatrical release and was not a financial success. 2006 brought more success in Farrell's career, as he appeared opposite Jamie Foxx in Michael Mann's action-crime film Miami Vice. The film received box office status grossing a total of US$ 163,557,986.00 in its worldwide revenues; Universal Pictures released it in over 3,000 theatres worldwide and it captured the top spot at the box office, grossing more than US$ 25.7 million in its opening weekend.
Farrell was next seen in Woody Allen's Cassandra's Dream, which premiered in 2007 and was distributed in the U.S. in early 2008. Farrell's next film, Martin McDonagh's In Bruges, opened the Sundance Film Festival in 2008; Farrell received his first Golden Globe nomination and win for his role as Ray, a hired hitman. Shortly thereafter he appeared in Kicking It, a documentary following six homeless men from different countries as they attempt to qualify for the Homeless World Cup. Farrell appeared on screen and provided narration. The film released simultaneously in theaters and television, airing on ESPN2 with a very short window to DVD release. Farrell received positive press for his involvement in the heartwarming true-life tale, and enthusiastic reviews for the two dramatic roles that preceded it.
On 11 January 2009, he won the Golden Globe award for Best Actor: Musical or Comedy for his role in In Bruges, in which he co-starred with Brendan Gleeson. The same year, he starred in Terry Gilliam's film The Imaginarium of Doctor Parnassus, alongside Christopher Plummer. He was one of the actors, along with Johnny Depp and Jude Law, who helped complete the late Heath Ledger's role after he died before filming ended. They all played "Imaginarium" versions of Ledger's character Tony. He also took an uncredited role as Tommy Sweet in Crazy Heart, alongside Academy Award-winning Jeff Bridges. 2010 saw the release of Ondine, a fantasy-drama directed by Neil Jordan, which stars Farrell as a fisherman.