O'Riordan's distinctive Irish lilt and yodel helped fuel the Cranberries' rapid rise in the early 1990s with global hits "Linger", "Dreams" and "Zombie".
Police said they were called to a hotel in Park Lane in the centre of the British capital at 0905 GMT on Monday and the death was being treated as "unexplained" as an investigation continues.
On Tuesday, a police spokeswoman confirmed the case was not being viewed as suspicious and had now been passed on to a coroner.
She released two solo albums during the hiatus before the band reformed in 2009.
The Cranberries recorded five albums with O'Riordan before they went on hiatus in 2003, the year before she launched her solo career.More news: Newly-discovered diamond among world's largest ever
She would later recall to the Irish Times how she wished "the moment would linger", but was disappointed when, on the next night out, she found he had fallen for another girl.
The singer had been in London to record a cover of the band's hit Zombie by the Los Angeles heavy rock band Bad Wolves. However, when they received a slip of mail that was wrongly addressed to The Cranberries, they decided they liked that better and just went with it.
Meawhile, Irish president, Michael D Higgins, called her death a "big loss", saying that her music with The Cranberries "had an enormous influence on rock and pop music in Ireland and internationally".
Although O'Riordan was known for being a hard-rocker who toured frequently, battled mental illness behind closed doors.
According to the managing director for Eleven Seven International, O'Riordan was looking forward to seeing him in the studio. "We hope we can still make her proud by sharing our version of "Zombie" with the world".
She had an ex-husband in former Duran Duran manager Don Burton, to whom she was married for 20 years before they got divorced in 2014, and they had three children together. She grew up in Ballybricken, which is actually in the Archdiocese of Cashel and Emly, but Limerick city, all of Limerick, held her very dear in its heart.
In a candid interview from 2013, the performer opened up about the traumatic time in her life, saying: "He [the abuser] used to masturbate me when I was eight-years-old". In May of previous year, The Cranberries were out on a European tour when the band had to cancel their remaining dates due to health issues O'Riordan was experiencing. "Her memory will live on", he added.