It's unclear what exactly Sinead O'Connor was expecting to get from Miley Cyrus in return for her "motherly advice," but it seems that a five-word tweet calling her crazy was definitely not it.

In a Facebook post published by O'Connor overnight, the singer threatens legal action against Miley if the latter doesn't quit comparing her to Amanda Bynes.

"Ms Cyrus has today posted tweets of mine which are two years old and which were sent when I was ill and seeking medical help," O'Connor wrote on Facebook. "She has done this in an attempt to deliberately cause me harm and hurt. I wish to confirm that I am quite well and kindly request people cease emailing me in the mistaken belief these are recent tweets. Ms Cyrus' lawyers will be contacted by mine regarding this matter."

Undaunted, Miley poked at O'Connor some more by tweeting, "Sinead. I don't have time to write you an open letter cause Im hosting & performing on SNL this week."

"So if youd like to meet up and talk lemme know in your next letter. :)," she added a moment later.

Miley's latest taunt prompted O'Connor to issue yet another open letter, her most stern to date:

I have no interest whatsoever in meeting you. You had plenty of time yesterday to abuse Amanda Bynes .. an entirely innocent party.. and myself.. who also did nothing to deserve your abuse.. along with every other sufferer of mental health problems and every person who suffered abuse at the hands of priests.
You can take five minutes today between g- string fuckin' changes to publicly apologise and remove your abusive tweets. If you do not then you don't give a shit who you mock and what damage you do by being so ignorant.
When you publicly apologise to Amanda and myself and all mental health sufferers as well as all who were abused by priests that will end the matter as far as I am concerned.
what you did yesterday was designed to damage me and my career and has caused me enormous distress and harassment and has potential to damage my career, since you deliberately gave the impression those tweets of mine were not two years old but reflect my current condition. If you cannot apologize I will have no choice but to bring legal proceedings against you since it is extremely hard to be given work when people think one is suffering from mental illness.
I have no interest in or desire to cause you trouble but if you do not apologise for having deliberately tried to cause me hurt and trouble personally and professionally I will have to bring pressure upon you.
When you end up in the psych ward or rehab I'll be happy to visit you.. and would not lower myself to mock you.
Be a proper woman and make the public apologies I have listed above. Your hosting SNL is a bullshit reason for not taking five minutes to do the right thing and your behaviour yesterday will rebound upon you very badly.
You have no business abusing Amanda Bynes or anyone else. How do you think you made her feel yesterday? How do you feel when your friend Britney Spears is mocked and humiliated for having had mental health problems? I know I personally want to bash those who treat her that way. If she is your friend and more importantly if you are a true friend to her.. you ought apologise for joining those who mock and humiliate women who have been too nice frankly, to manage the music business without sensibly losing their minds.
Cease behaving in an anti-female capacity. You will become the victim of it shortly. Soon it will be you the media 'crazy' .. and you will not enjoy it.. and you will appreciate people (like myself) standing up for you. Which I will be happy to do.. if you earn my respect today by apologising publicly.

Miley will no doubt be publicly apologizing any minute now, just as soon as she puts on some clothes.

[photos via Twitter, Facebook]