An 18-year-old woman and her younger twin sisters have been killed after a "violent and traumatic incident" at a home in Dublin.
Police officers were called to a property in the Tallaght area of the city at around 12.30am on Sunday.
Two children and a teenager were taken by ambulance to Children's Health Ireland (CHI) in Crumlin, but all three were later pronounced dead.
A teenage boy was also taken from the scene to Tallaght Hospital with serious injuries which are not thought to be life-threatening.
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Gardai described it as a "violent, challenging and traumatic incident" but have not yet provided further details about what happened.
The Garda Armed Support Unit used non-lethal devices to arrest a man in his early 20s at the scene.
He is currently detained at Tallaght Garda Station under Section 4 of the Criminal Justice Act 1984.
The mother of the children, who was not injured at the scene, was also taken to CHI and is currently receiving medical treatment.
Gardai said everyone involved is believed to be known to each other and officers are not looking for anyone else in connection to the incident.
The scene at the Rossfield Estate remains sealed off and a forensic examination will take place on Sunday by the Garda Technical Bureau.
It's believed the teenager was 18-years-old and her two sisters - twins - were aged 8.
The Coroner's Office and the Office of the State Pathologist have been informed and post-mortem examinations are due to take place. A family liaison officer has been appointed and will continue to support those involved, Gardai added.
Locals described how they were woken up by the wailing of police sirens and a helicopter in the early hours of the morning.
"I was awake all night listening to the sirens and chopper. God love them all," one woman said.
Another nearby resident wrote on Facebook: "Terribly sad. I could hear helicopter and police sirens last night."
South Dublin County Council councillor Charlie O'Connor described the incident as a "really sad event". "Over here in Rossfield you can get that sense...that people are genuinely shocked," he told Sky News.
"We expected this incident to have an effect on the community not only here in Brookfield, but in the wider Tallaght area."
There is a large police presence in the area while a full forensic examination of the scene is under way.
The upstairs windows of the property were open and smashed while blood stains could be seen on the window frames.
A smashed downstairs window also remained open while forensic officers examined the front garden.
A black car outside the property was also a focal point of the investigation.
Forensic officers removed a number of items from the scene, including a shoe, small bag and black coat which were placed in forensic bags.
Flowers have also been left at the scene. One floral tribute said: "Fly high little angels. Our hearts are broken for you all."
The streets surrounding the property remained busy with children playing nearby.
Helen McEntee, Ireland's justice minister, said: "The killing of three young family members is a tragedy, so awful and heart-breaking. It is difficult to comprehend.
"My thoughts are with the family and the local community as they deal with this unimaginable crime.
"Gardai and other services are supporting them at this difficult time."
Dermot Richardson, Sinn Fein councillor for the Tallaght area, called for welfare support to be put in place for the family, local community and schoolchildren.
He also called for people to stop sharing video footage of the incident on WhatsApp and social media sites.
"There are videos being shared on WhatsApp. I would ask people not to share it out of respect for the family and the local community as well," he added.
"If you have footage, please pass on to the gardai."
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