Sinéad O'Connor was born in Dublin and was named after Sinéad de Valera, wife of Irish President Éamon de Valera and mother of the doctor presiding over the delivery, and Saint Bernadette of Lourdes. She is the middle of five children, sister to Joseph, Eimear, John, and Eoin. Joseph O'Connor is now a notable novelist.

Her parents are Sean O'Connor, a structural engineer later turned barrister, and Marie O'Connor. The couple married young and had a troubled relationship, separating when Sinéad was eight. The three eldest children went to live with their mother, where O'Connor claims they were subjected to frequent physical abuse. Her song "Fire on Babylon" is about the effects of her own child abuse, and she has consistently advocated on behalf of abused children. Sean O'Connor's efforts to secure custody of his children in a country which routinely gave custody to the mother and prohibited divorce motivated him to become chairman of the Divorce Action Group and a prominent public spokesman. At one point, he even debated his own wife on the subject on a radio show.

In 1979, O'Connor left her mother and went to live with her father and his new wife. However, her shoplifting and truancy led to her being placed in a reform school at age 15, the Grianán Training Centre run by the Sisters of Our Lady of Charity. In some ways, she thrived there, especially in writing and music, but she also chafed under the imposed conformity. Unruly students there were sometimes sent to sleep in the adjoining nursing home, an experience of which she later commented, "I have never — and probably will never — experience such panic and terror and agony over anything."

One of the volunteers at Grianan was the sister of Paul Byrne, drummer for the band In Tua Nua, who heard O'Connor singing "Evergreen" by Barbra Streisand. She recorded a song with them called "Take My Hand" but they felt that at 15, she was too young to join the band.

In 1983, her father sent her to Newtown School, an exclusive Quaker boarding school in Waterford, an institution with a much more permissive atmosphere than Grianan. With the help and encouragement of her Irish language teacher, Joseph Falvey, she recorded a four-song demo, with two covers and two of her own songs which would later appear on her first album.

Through an ad she placed in Hot Press in the summer of 1984, she met Columb Farrelly. Together they recruited a few other members and formed a band called Ton Ton Macoute, named for the zombies of Haitian myth. In the autumn, the band moved to Waterford briefly while O'Connor attended Newtown, but she soon dropped out of school and followed them to Dublin, where their performances received positive reviews. Their sound was inspired by Farrelly's interest in witchcraft, mysticism, and world music, though most observers thought O'Connor's singing and stage presence was the band's driving force.

On 10 February 1985, O'Connor's mother was killed in an automobile accident. O'Connor was devastated despite her strained relationship with her mother. Soon afterward she left the band, which stayed together despite O'Connor's statements to the contrary in later interviews, and moved to London.

O'Connor's time as singer for Ton Ton Macoute brought her to the attention of the music industry, and she was eventually signed by Ensign Records. She also acquired an experienced manager, Fachtna O'Ceallaigh, former head of U2's Mother Records. Soon after she was signed she embarked on her first major project, providing the vocals for the song "Heroine", which she cowrote with U2's guitarist The Edge for the soundtrack to the film Captive. O'Ceallaigh, who had been fired by U2 for complaining about them in an interview, was outspoken with his comments about music and politics, and O'Connor began to adopt the same habits; she made controversial comments about the IRA and even directed negative remarks towards U2.

Things were contentious in the studio as well. She was paired with veteran producer Mick Glossop, whom she later publicly derided. They had differing visions regarding her debut album and four months of recordings were scrapped. During this time she became pregnant by her session drummer John Reynolds (who went on to drum with the band Transvision Vamp) and the record company pressured her to get an abortion. Thanks largely to O'Ceallaigh's persuasion, the record company allowed O'Connor, 20 years old and by then seven months pregnant, to produce her own album.

O'Connor's first two albums (1987's The Lion and the Cobra and 1990's I Do Not Want What I Haven't Got) gained considerable attention and mostly positive reviews. She was praised for her voice and her original songs. She was also noted for her appearance: her shaved head, often angry expression, and sometimes shapeless or unusual clothing.

The Lion and the Cobra was not embraced by the pop mainstream on a large-scale basis, but the album did eventually hit gold record status and earned a Best Female Rock Vocal Performance Grammy nomination. The single "Mandinka" was a big college radio hit, and "I Want Your (Hands on Me)" received both college and urban play in a remixed form that featured rapper MC Lyte. In her first US network television appearance, O'Connor sang "Mandinka" on Late Night with David Letterman in 1988. The single "Troy" was also released as a single in the UK and Ireland. A club mix of "Troy" would become a major US dance hit in 2002.

The following year, O'Connor joined The The frontman Matt Johnson as a guest vocalist on the band's album Mind Bomb, which spawned the duet "Kingdom of Rain."

The album I Do Not Want What I Haven't Got featured Marco Pirroni and Kevin Mooney, of Adam and the Ants fame, and contained her international breakthrough hit "Nothing Compares 2 U", a song written by Prince and originally recorded and released by a side project of his, The Family. Aided by a memorable and well received video by John Maybury which consisted almost solely of O'Connor's face as she performed the song, it became a massive international hit, reaching #1 in several countries. In Ireland it hit the top spot in July 1990 and remained there for 11 weeks; it is the eighth most successful single of the decade there. It had similar success in the UK, charting at #1 for three weeks, and in Germany (#1 for 11 weeks). In Australia, it reached #1 on the Top 100. It also claimed the #1 spot on the Hot 100 chart in the USA. She also received Grammy nominations including Record of the Year and Best Female Pop Vocal Performance. She eventually won the Grammy for Best Alternative Music Performance, but boycotted the award show.

Public Enemy's Hank Shocklee remixed the album's next single, "The Emperor's New Clothes," for a 12-inch that was coupled with the Celtic funk of "I Am Stretched On Your Grave." Pre-dating but included on I Do Not Want What I Haven't Got was also "Jump in the River," which originally appeared on the Married to the Mob soundtrack; the 12-inch version of the single had included a remix featuring performance artist Karen Finley.

In 1990, she joined many other guests for former Pink Floyd member Roger Waters' massive performance of The Wall in Berlin. (In 1996, she would guest on Broken China, a solo album by Richard Wright of Pink Floyd.) In 1991 her take on Elton John's "Sacrifice" was acclaimed as one of the best efforts on the tribute album Two Rooms: Celebrating the Songs of Elton John & Bernie Taupin.

In 1990, she contributed a cover of "You Do Something to Me" to the Cole Porter tribute/AIDS fundraising album Red Hot + Blue. This was followed by the release of Am I Not Your Girl?, an album of standards and torch songs that she had listened to while growing up. The record lost her much of the commercial momentum her career had built up until then. Also in 1992 she contributed backing vocals on the track "Come Talk To Me", and shared vocals on the single "Blood of Eden" from the studio album Us by Peter Gabriel.

After spending nine years dividing her time between London and Los Angeles, O'Connor returned to her home town of Dublin in late 1992 to live near her sister and focus on raising her son Jake, then six years old. She spent the following months studying Bel Canto singing with teacher Frank Merriman at the Parnell School of Music. In an interview with The Guardian published 3 May 1993 she reported that her singing lessons with Merriman were the only therapy she was receiving, describing Merriman as "the most amazing teacher in the universe."

The 1993 soundtrack to film In the Name of the Father featured "You Made Me the Thief of Your Heart," with significant contributions from U2 frontman Bono.

The more conventional Universal Mother (1994) did not succeed in restoring her mass appeal. She toured with Lollapalooza in 1995, but dropped out when she became pregnant. O'Connor was replaced on the bill by Elastica. The Gospel Oak EP followed in 1997, and featured songs based in an acoustic setting. It too, did not recapture previous album successes.

In 1994 she appeared in A Celebration: The Music of Pete Townshend and The Who, also known as Daltrey Sings Townshend. This was a two-night concert at Carnegie Hall produced by Roger Daltrey of The Who in celebration of his 50th birthday. A CD and a VHS video of the concert were issued in 1994, followed by a DVD in 1998.

Faith and Courage was released in 2000, including the single "No Man's Woman," and featured contributions from Wyclef Jean of the Fugees and Dave Stewart of Eurythmics. On the eve of its release, O'Connor came out as a lesbian, and then retracted the statement.

Her 2002 album, Sean-Nós Nua, marked a departure in that O'Connor interpreted or, in her own words, "sexed up" traditional Irish folk songs, including several in the Irish language. In Sean-Nós Nua, she covered a well-known Irish folk song, "Peggy Gordon", interpreted as a song of lesbian, rather than heterosexual, love. In her documentary, Song of Hearts Desire, she stated that her inspiration for the song was her friend, a lesbian who sang the song to lament the loss of her partner.

In 2003, she contributed a track to the Dolly Parton tribute album Just Because I'm a Woman, a cover of Parton's "Dagger Through the Heart". That same year, she also released a double album, She Who Dwells in the Secret Place of the Most High Shall Abide Under the Shadow of the Almighty. The album contained one disc of demos and previously-unreleased tracks and one disc of a live concert recording. Directly after the album's release, O'Connor announced her retirement from music.

Collaborations, a compilation album of guest appearances, was released in 2005 - featuring tracks recorded with Peter Gabriel, Massive Attack, Jah Wobble, Terry Hall, Moby, Bomb The Bass, The Edge, U2, and The The. Numerous other collaborations from throughout her career are yet to be compiled.

Ultimately, after a brief period of inactivity and a bout with fibromyalgia, her retirement proved to be short-lived - O'Connor stated in an interview with Harp that she only intended to retire from making mainstream pop/rock music, and after dealing with her fibromyalgia, chose to move into other musical styles. The reggae album Throw Down Your Arms appeared in late 2005 and was greeted with very enthusiastic reviews, critics considering it one of O'Connor's best albums. It was based on the Rastafarian culture and lifestyle, O'Connor having spent time in Jamaica in 2004. She performed the single "Throw Down Your Arms" on The Late Late Show in November. She also made comments critical of the war in Iraq and the role played in it by Ireland's Shannon Airport.

On 8 November 2006, O'Connor performed seven songs from her upcoming album Theology at The Sugar Club in Dublin. Thirty fans were given the opportunity to win pairs of tickets to attend along with music industry critics. The very well received performance was recorded and filmed for future release on her website.

O'Connor released two songs from her album Theology to download for free from her official website: "If You Had a Vineyard" and "Jeremiah (Something Beautiful)". The album, a collection of covered and original Rastafari spiritual songs, was released in June 2007. The first single from the album, the Rice/Lloyd Webber classic "I Don't Know How to Love Him", was released on 30 April 2007. To promote the album, O'Connor toured extensively Europe and North America. She also appeared on two tracks of the new Ian Brown album The World Is Yours, including the anti-war single "Illegal Attacks".

She was nominated in the Best Irish Female category for Ireland's celebrated Meteor Awards 2008. O'Connor collaborated with Irish based funk rock band Republic Of Loose on a new version of Curtis Mayfield's We People (Who Are Darker Than Blue). They performed the single at the Meteor Awards 15 February 2008.

O'Connor is said to be working on a new, yet-untitled album. She toured Europe during the summer of 2008, performing mainly Theology material in an intimidate, acoustic setting.

Recently O'Connor performed "Troy" live for the first time since 1990, along with "Nothing Compares 2 U" and "Dark I Am Yet Lovely". The set was part of the Night of the Proms concert series in Antwerps, Belgium, and has met with enthusiastic reviews.