A rep for London's Southwark Coroner's Court tells TMZ ... she passed away from "natural causes" and "the coroner has therefore ceased their involvement in her death."

As we reported, cops in London responded to a report of an unresponsive woman at a residence ... arriving to find the 56-year-old singer already dead. Attempts were made to revive her, but they were unsuccessful.

Sinéad's 17-year-old son, Shane, died by suicide in early 2022 just 2 days after going missing from the hospital while on suicide watch. After his death, Sinéad called her son "the very light of my life" ... begging for him to be at peace.

She expressed more grief about Shane less than 2 weeks before her death, saying on social media, "He was the only person who ever loved me unconditionally. I am lost in the bardo without him."

Sinéad has also struggled with her mental health in the past, telling Dr. Phil in 2017 she'd attempted suicide 8 times in a year.

She also was reported missing the year before, allegedly threatening to jump off a bridge -- but Sinéad was found safe and said the suicide claims were BS.

The singer's seen a ton of love from the community since her death ... including from Conor McGregor, who had her sing during his 2015 walkout at UFC 189 in Las Vegas. He told TMZ, "The world has lost a beautiful voice and an even more beautiful person."

Pink and Brandi Carlile also paid tribute to her, jumping on stage together to perform a moving rendition of Sinéad's hit song, "Nothing Compares 2 U."