At about 12.30am on Sunday, gardaí were informed of a violent incident taking place at a house in the Rossfield Estate in Tallaght.
An 18-year-old woman and her brother and sister, twins aged eight, were taken from the scene by ambulance to hospital in Crumlin, where they were pronounced dead a short time later.
A teenage boy (14) was taken to Tallaght Hospital with serious but non-life threatening injuries.
A man in his early 20s was arrested at the scene following an intervention by armed gardaí involving the discharge of less than-lethal devices.
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, was also taken to Crumlin hospital and is currently receiving medical treatment.
All people involved in the incident are believed to be known to each other. Gardaí said they were not looking for anyone else in relation to the incident.
The scene in Rossfield Estate remains sealed off and a forensic examination will begin later on Sunday by the Garda Technical Bureau.
Gardaí in Tallaght have established an incident room and said all circumstances were being investigated.
Officers described it as a violent, challenging and traumatic incident. An Garda Síochána said it has put in place welfare support for all staff who attended.
Fianna Fáil councillor Charlie O’Connor, who was at the scene on Sunday morning, said there is widespread shock in the community.
“I don’t know how to describe it. I was getting calls from early morning with people bringing it to my attention,” Mr O’Connor said.
“People are only now beginning to come out of their house. I was at the Catholic Church a while ago and a lot people hadn’t heard, but the news was getting through.
“There is deep shock, people are really, really upset, any event like this would cause shock but the fact that it involves children.
“It’s a really shocking event for this community, and indeed for the wider Tallaght community and elsewhere. This will affect many, many people.
“I was talking to the gardaí and they told me that their colleagues who were on last night were all deeply shocked.
“It is terrible. We have to think of the family, think of their neighbours, think of the wider community.
“Attempts are being made to support the local community and the local Brookfield Youth and Community Centre has just confirmed to me that they’re opening up now for the rest of the day.
“People are welcome to come and grieve and have a cup of tea and talk about their grief.
“I really find it shocking to think three children of that age are no longer with us and won’t be going to school in the morning.”
There is a large gardaí 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.
The streets surrounding the property remained busy with children playing nearby.
The killing of three young family members is a tragedy so awful and heartbreaking it is difficult to comprehend
My thoughts are with the family and the local community as they deal with this unimaginable crime
Gardaí and other services are supporting them at this difficult time
— Helen McEntee TD (@HMcEntee) September 4, 2022
The Minister for Justice, Helen McEntee, 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.
“Gardaí and other services are supporting them at this difficult time.”
Dermot Richardson, Sinn Féin 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 gardaí.” – Additional reporting: PA