The artist, who is best known for her 1990 hit version of the song Nothing Compares 2 U, said she had changed her name to Shuhada'. Announcing her conversion to Islam, Sinéad O'Connor further thanked fellow Muslims for extending their support. Her display name on Twitter has changed to reflect the name change.
The singer also gushed about wearing her "first hijab", which was given to her by a friend.
She wrote: "Here is my 1st attempt at singing the Azan". And I'm an ugly old hag.
In 2007, Sinead said she considered herself a Christian, and added: "God loves everybody".
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But her career began to falter after she tore up a picture of the late Pope John Paul II while appearing as a musical guest on United States television in 1992.
In April 1999, she was then ordained a priest by the Catholic Latin Tridentine Church in Lourdes, France and adopted the name Mother Bernadette Mary.
Brand sent a supportive message to the 50-year-old on YouTube, saying that following a public breakdown she needed "connection, meaning, purpose, love and a bit of time, really".
The Catholic Church dismissed the ordination of the singer at the time as "bizarre and absurd".
In December 2015, she wrote on Facebook that she had been detained in hospital for a mental health evaluation.
She has also openly spoken about past abortions - a contravention of Canon Law.