The abuse had left “profound mental scars” on his older daughter, now 18, the father told the Daily Mail.
He said McCartney first made contact with his “outgoing, intelligent, funny” daughter on Snapchat, which she had used to keep in touch with school friends.
He then spent months building her trust so that she did not think she was talking to anyone but another girl of her age.
“Eventually they got to a point where they exchanged photos,’ her father told the Daily Mail.
“So she sent him a nude picture. And once he had that, he obviously had all the power, and she was playing by his rules.”
The father said McCartney used that first picture to blackmail and manipulate his daughter further, including by having her also send photos of her younger sister who was 10 at the time.
The abuse continued when McCartney tried to get the girls’ 17-year-old cousin to send photos of herself.
However, the cousin immediately told the father and his wife and they went to police.
While McCartney’s identity was not yet known at that time, they quickly found out the hunt for him was part of an investigation stretching across the world.
The case brought against him in Belfast Crown Court focused on 70 of his victims, even though he targeted as many as 3500, the BBC reported.
That included US girl Cimarron Thomas, who he messaged when she was 12.
After getting a photo of her, he demanded more, threatening to send the photo of her to her dad if she didn’t do what he wanted, the Daily Mail reported.
When he kept pursuing her and told her to include her little sister, Cimarron begged him to stop and told him she would kill herself.
McCartney replied “goodbye and goodluck” and put up a countdown clock, according to the Daily Mail and BBC.
Cimarron then took her own life in 2018.
Justice O’Hara told the Belfast Crown Court he did not sense shame or remorse in McCartney.
McCartney’s crimes stretched from 2013 to 2019 before he made his first appearance in court that year.