Writing on Twitter, the former Westlife singer called on O'Connor to shrug off any criticism.
"Message to sinead o connor. Ignore what anyone says. Its your life and you don't have to explain yourself to anyone," McFadden said.
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The 'Nothing Compares 2 You' singer left husband Barry Herridge after 16 days due to "intense pressure" from his circle of friends and family.
They married at Las Vegas in a drive-through wedding on 8 December.
She wrote a letter to fans explaining candid details of her relationship with Herridge.
