Just days after the 50-year-old singer broke down in tears in an emotional video shared on her Facebook page, in which she admitted she is suicidal, Fiona, 39, shared her own video message in which she sent her support to Sinead.
She said: ''I want you to know that you are my hero as well. I just saw the video of you and I don't want you to feel like that. I don't want you to feel like that. You've given so much. And I wish I could be there. I wish I could be of some use to you... I'm your friend, that's all I want to say, and you're my hero.''
Singer-songwriter Fiona added a video of O'Connor's performance at the 1989 Grammy Awards.
Fiona's message came just days after Sinead sparked fears for her life after admitting three mental illnesses have made her ''suicidal'' and her doctor is the only thing ''keeping [her] alive''.
In the 12-minute clip, which she captioned ''#OneOfMillions'', she said: ''I am now living in a Travelodge motel in the a**e end of New Jersey, I've got a f***ing kidney stone.
''I'm all by myself. And there's absolutely nobody in my life except my doctor, my psychiatrist - the sweetest man on earth, who says I'm his hero - and that's about the only f***ing thing keeping me alive at the moment ... and that's kind of pathetic.
''I want everyone to know what it's like, that's why I'm making this video.
''Mental illness, it's like drugs, it doesn't give a s**t who you are, and equally what's worse, it's the stigma, it doesn't give a s**t who you are.
''Suddenly all the people who are supposed to be loving you and taking care of you are treating you like s**t. It's like a witch hunt.
''Three f***ing illnesses made me suicidal ... my whole life is revolving around just not dying.
''And I'm not going to die, I'm not going to die but still this is no way for people to be living.''
Sinead - who has four children from different relationships - revealed on Facebook in 2015 she had taken an overdose at a hotel in Ireland but she was later found safe.
He also slapped a jet-lagged Taylor Hawkins.
The acclaimed performer had just cancelled touring due to a battle with cancer.