The Nothing Compares 2 U singer is believed to be still receiving treatment in hospital after posting a message online on Sunday (29Nov15) stating she had taken an overdose.
Sinead, who has a history of mental health issues, wrote another reference to suicide in a post on Wednesday (02Dec15) resulting in the suspension of her Facebook.com account, but Boomtown Rats star Bob has now thrown his support behind his close pal.
He tells Ireland's Radio Nova, "I didn't know about it but she's a great friend and one of the great Irish artists... She's just a great, great spirit and sometimes that's exhausting. I'm sorry to hear (about her latest health issues) but she'll come out of it. She'll be back."
Sinead was taken to a hospital in Ireland on Sunday after her online message sparked fears for her health, and officials later confirmed she was "safe and sound" and was receiving treatment.