Miley had compared Sinead to Amanda Bynes, who is currently seeking psychiatric help, by posting a string of tweets from an account previously run by Sinead when she was unwell, captioning it: "Before Amanda Bynes.... There was...."
Sinead O'Connor and Miley Cyrus
It came after the 'Nothing Compares 2U' singer wrote her an open letter over her recent controversies, telling her that 'the industry doesn't give a s**t about you' and 'will prostitute you for all you are worth'.
Sinead has now posted a fresh letter to Miley on Facebook, branding her tweets 'anti-female', 'dangerous', 'irresponsible', and 'f***ing stupid'.
"Miley… Really? Who the f**k is advising you? Because taking me on is even more f**kin' stupid than behaving like a prostitute and calling it feminism. You have posted today tweets of mine which are two years old, which were posted by me when I was unwell and seeking help so as to make them look like they are recent," Sinead wrote.
"In doing so you mock myself and Amanda Bynes for having suffered with mental health issues and for having sought help. I mean really really… who advises you? [H]ave you any idea how stupid and dangerous it is to mock people for suffering illness? You will yourself one day suffer such illness, that is without doubt.
"The course you have set yourself upon can only end in that, trust me. I am staggered that any 20 yr old woman of the 21st century could behave in such a dangerous and irresponsible manner as to not only send the signal to young women that its ok to act like prostitutes but also to the signal that those who have suffered or do suffer mental health problems are to be mocked and have their opinions invalidated.
"Have you no sense of danger at all? or responsibility? Remove your tweets immediately or you will hear from my lawyers. I am certain you will be hearing from all manner of mental health advocacy groups also. It is not acceptable to mock any person for having suffered."
She continued: "It is most unbecoming of you to respond in such a fashion to someone who expressed care for you. And worse that you are such an anti-female tool of the anti-female music industry.
"I hope that you will apologise to Amanda Bynes and to any person who has been wounded by your mockery of those who have suffered. And I hope that you will wake up and understand that you in fact are a danger to women.
"Furthermore you posted a photo of me tearing the pope's photo .. as if to imply insanity.. by doing so all you have achieved is to expose your staggering ignorance. I suggest you read The Philadelphia Report, The Boston Report, all the reports which will illuminate for you why that action of mine remains sane and valid.
"By mocking it you mock every child who suffered sexual abuse at the hands of priests and had it covered by the Vatican. You could really do with educating yourself, that is if you're not too busy getting your tits out to read."
After the reading the letter, Miley took to Twitter once again, insisting she was too busy to reply to her properly, and invited her to meet up.
"Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week," she tweeted.
"So if you'd like to meet up and talk lemme know in your next letter. :)"
Sinead, over to you… again.
If you've been affected by the issues in this article, please call the Samaritans on 08457 90 90 90.