"It is just tragic news. It is just unbelievable that it has actually happened," Raymond McGuinness, the former partner of victim Marion Graham, said.
"The family are still trying to come to grips with what happened in Kusadasi."
Marion and her friend Cathy Dinsmore were stabbed to death in a wooded area of Izmir, where their bodies were found on Friday.
The Turkish boyfriend of Marion and Raymond's daughter, Shannon, has been arrested and is understood to have already confessed to the killings.
Much of the detail of the case remains speculative, amid confusion over the accused's age. Recep Chetin (also known as Alex) was initially thought to be 17, but could actually be in his 20s.
One line of enquiry is that he murdered the women because Shannon's family wouldn't allow the couple to be married.
Alex knew that he was going to be losing Shannon - she was coming home back to Ireland once again and only he actually knows what’s actually happened.
Raymond McGuinness
Shannon's dad, who just flew back from Turkey - where he had to formally identify Marion's body - with Shannon on Monday, met her boyfriend last year and admits he had concerns.
"He had said he was 17," he explained. "I didn't mind as long as he treated my daughter with respect and he was good to her and he was kind - which she expressed.
"But I did express to Marion that I wasn't too happy with it and Marion said: 'Listen, it'll blow over. It'll just be a holiday romance.' But obviously they kept in touch."
But Mr McGuinness added that the marriage "would never have happened" and that Shannon was about to go back to school after the holidays.
It is understood Shannon was on a boat trip when the murders happened and that she tried to phone her mum on the way back, but there was no answer.
Her dad told UTV the teenager made her way back to the villa in Kusadasi, where she met Alex and saw his hands were cut.
"He'd said that Marion and Cathy had been abducted and that he was able to get away and he wanted to go to hospital to get his injuries treated," Mr McGuinness said.
Shannon went to hospital with him because she believed him, but later noticed he had changed his clothes and told him to go to the police.
If Recep Chetin is tried in an adult court, he could face up to 50 years in prison for the murders of Marion Graham and Cathy Dinsmore.
UTV’s Marc Mallett in Turkey
"It's very hard to understand the motive behind it," Mr McGuinness said.
Telling UTV his daughter's life had been "changed forever", Shannon's dad said she was his main concern now.
"She's only 15 years of age," he said. "Not only has she lost her mum, but she's lost her best friend."
Mr McGuinness didn't want to appear on television, but he did want to pay tribute to his former partner.
"Marion was a wonderful woman," he said. "She was the life and soul of the party. She was a very kind woman. She just thought the world of Shannon."
He also expressed his condolences to the Dinsmore family, describing Marion and Cathy as "the best of friends".
The bodies of both women will arrive in Dublin on Wednesday afternoon.