Witnesses told a coroner's inquest at Westminster Coroner's Court on Thursday that O'Riordan was found submerged in her bathtub at a London hotel in January.
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The 46-year-old didn't leave a note and there was no evidence of self-harm.
Coroner Shirley Radcliffe ruled O'Riordan's death an accident.
The Cranberries formed in the Irish city of Limerick at the end of the 1980s and had international hits in the '90s with songs including "Dream," ''Linger" and "Zombie."
Celebrities and notable figures who have recently passed away
The band split up in 2003 but reunited several years later. The Cranberries released the acoustic album "Something Else" in 2017 and had been due to tour Europe and North America. The tour was cut short because O'Riordan was suffering from back problems.
In 2014, O'Riordan was accused of assaulting three police officers and a flight attendant during a flight from New York to Ireland. She pleaded guilty and was fined 6,000 euros ($6,600).