The girl's aunt, who answered the door at her house in the Drumcairn Avenue area of Tallaght, said Melanie was in the wrong place at the wrong time.
"She was an innocent child, she had nothing to do with anything," she said.
"She didn't deserve it. She wasn't the target. She was never involved in anything in her life."
Ms McCarthy was sitting in the back passenger side seat of a silver-coloured Nissan car with three men in their 20s, one of them her boyfriend, when the gunman struck at around 10.35pm.
At least one shot was fired from the Hyundai into the car, shattering the side window and hitting the teenager in the head.
She was two weeks from her 17th birthday.
The driver of the Nissan rushed her to Tallaght Hospital but despite intensive efforts to save her, she was pronounced dead around 1am.
Her body was removed to the mortuary for a post-mortem examination by the state pathologist.
Ms McCarthy was thought to have been living in Tallaght for some time but had previously been living in the Gortboy area of Newcastle West in Co Limerick.
The Hyundai, which fled the estate at speed, was later found abandoned in the CityWest area, near Brown's Barn restaurant.
It was not burned out and detectives are to carry out forensic tests on the vehicle.
That scene and the scene of the shooting have been sealed off for tests by Garda technical experts.
Justice Minister Alan Shatter said the killing was an appalling and totally barbaric tragedy.
"This is an absolutely horrific incident. These type of incidents will not be tolerated," he said.
"Gun law on the streets of Tallaght, or any other part of the city or county of Dublin or anywhere else in the country, is entirely unacceptable.
"Unfortunately, we have groups of people in the state who have no respect for human life and quite frankly, we have too many people accessing guns."
Investigators said the Hyundai used in the killing had been stolen and was fitted with false 07 D registration plates.
The vehicle made its way from the scene of the shooting along Brookview Avenue and into Fortunestown Lane before being abandoned near Brown's Barn.
It is thought the escape journey may have taken just five minutes, and Gardai believe witnesses may have noticed the killer or killers during the getaway.
They have appealed for anyone who saw the black SUV to get in contact.
Chief Superintendent John Manley, who is leading the inquiry, described the killing as a particularly heinous crime.
"I would appeal to the community to reflect on this and to take a step back and to provide whatever assistance they can give us to investigate this crime," he said.
Asking for witnesses to come forward, Chief Superintendent Manley added: "I want to reassure the public that we will give them every support that we can, but likewise I hope the public will give us the support that we need."
No one else was injured in the attack but the three other men in the Nissan car were left traumatised, gardai said.
Ballistic tests have yet to be completed, but detectives suspect a shotgun was used. It is thought it may have been fired more than once.
House-to-house inquiries are being carried out in the area.
- AAP
