Turkish waiter Recep Cetin stabbed both women more than 15 times, it has emerged.
He told Turkish police he had been "blinded by love" as he admitted murdering his girlfriend's mother and her best friend.
Recep (17) is understood to have attacked Marion Graham (53) first before hunting down Kathy Dinsmore (53), whose body was discovered 20 metres away.
He lured the two 53-year-old women to a wooded area near Izmir, 75 miles north of the resort of Kusadasi, where they were staying, under the pretext of shopping trip, while Shannon went on a boat trip with a friend.
He viciously stabbed the women at least 15 times around their throats and chests in a frenzied assault last Thursday.
It appears Kathy witnessed the attack on her pal before making a desperate bid for freedom.
Cetin is understood to have disguised his actions by pretending they had been kidnapped.
Witnesses saw him lie outside the Irish women's holiday home, where he had also been staying for the past three months, feigning unconsciousness for a few hours.
When he rose, he said that he was going to the hospital to treat his hand -- which he claimed had been badly slashed by the kidnappers.
"We saw him lying in the dirt on Thursday afternoon," his Turkish neighbours said.
"We went over to him and offered to call an ambulance but he came up and told us he'd take a taxi to hospital. On his way he dumped a bag in the rubbish bins. At that point, we had no idea what he'd done."
When Shannon returned home, he told her that three men had jumped out of a van and abducted her mum and Kathy but as he failed to explain his return to the rented villa, the Irish teenager became suspicious and contacted police.
While he was questioned, Cetin revealed that he had hoped to wed Shannon who he had been seeing for two years.
However, Marion and her friend Kathy, from Newry, Co Down, were opposed to the idea, as the girl was underage.
"They stole my dream -- that's why I killed them," he reportedly said, explaining that he loved Shannon like "crazy" and that he had to "eliminate" "obstacles" that were keeping them apart
The alleged killer -- who was known in the area as Alex -- was described as "obsessive, controlling and manipulative" by Shannon's friends, and neighbours recalled hearing many arguments from the townhouse.
Marion's former partner and Shannon's dad, Raymond McGuinness said that Marion "went ballistic" when she heard that Cetin had kicked their daughter during a fight.
She told him to keep away from the quiet girl but he threatened to kill Marion is she kept in between them.
Families of the two Irish women were in the courthouse this morning for Cetin's first full hearing.
It is expected that the Turkish teenager will be formally charged in the coming days.
Authorities are concerned that he may be older than his papers state as Turkish parents do not always officially register their children's birth so that they may avoid compulsory national service in the military for a few years.
If Cetin is 20 or 21 years old, as believed, he could be tried in an adult court.
The death penalty was abolished in Turkey in 2002 but he could still spend the rest of his life in prison if convicted of murder. Otherwise, the maximum sentence he could get as a minor is 24 years.
Meanwhile brave Shannon went back to the holiday townhouse with her brother David, and father Raymond to collect her belongings yesterday.
The family made no comment as they gathered laptops and suitcases before heading to a hotel half a mile away.