It's really none of her business what Miley Cyrus gets up to so why she felt the need to write that letter and make it public I have no idea.
It wasnt an unprovoked attack. Miley Cyrus had pointlessly and stupidly retweeted two year old tweets from Sinead O Connor when Sinead had been crying out for help during a manic episode. It was insensitive and thoughtless of Miley, and drew a lot of unwanted attention to O'Connor (in the form of press hounding) for a reaction. Sinead was telling her off publicly and so she should. PLUS she ripped off Sinead O Connors 'Nothing Compares to U' video and Sinead was getting hounded by newspapers by that, too for a reaction....
i think Sinead O Connor reacted with dignity and lots of common sense in her dressing down of Cyrus - she also mentioned a few times that she thinks Cyrus is talented and doesnt need to get her tits out. Its a caring and precautionary message. Good on Sinead!!