Farrell was born in Castleknock in Dublin, he is the son of Rita,a homemaker, and Eamon Farrell who operates a health food store and was a footballer who played for Shamrock Rovers FC. Farrell has three siblings, two sisters, Claudine and Catherine, and a brother, Eamon Óg.
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 and dated Angie Miller. Farrell auditioned for the Irish group Boyzone when he was still unknown. But he dropped out and was cast in the part of Danny Byrne on 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 lead role 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).
late 2003, Farrell starred as a criminal who plots a bank heist with Cillian Murphy in the comedy Intermission, which held the record as highest-grossing Irish independent film in Irish box office history until 2006. In 2004, Farrell appeared in several independent films that received only a limited theatrical release in most countries, including A Home at the End of the World, which received some positive reviews. Farrell 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 appeared in the title role of Alexander the Great in Oliver Stone's 2004 biopic Alexander, which, while receiving some favorable reviews internationally, received mostly mediocre and negative reviews in the United States. It was marked by controversy for openly portraying the ancient conqueror as bisexual, and among some historians for its portrayal of the ancient Persians, though others praised it for its accuracy in these regards as well.
His next film was 2005's Academy Award-nominated The New World, also a historical epic that was met with mixed reviews.
The New World was followed by Ask the Dust, a romance film set in period Los Angeles co-starring Salma Hayek. 2006 brought more success in Farrell's career, as he appeared opposite Jamie Foxx in Michael Mann's action-crime film Miami Vice.
Farrell married English actress Amelia Warner in July 2001 and divorced her four months later.
Farrell has a son, James (born September 12, 2003), with American model Kim Bordenave, from whom he has since separated.