A teenager and her two younger sisters died after an incident at a house.
Officers were called to the property in the Rossfield Estate in Tallaght 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.
The Garda Armed Support Unit used less-than-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.
Gardai in Tallaght have established an incident room and all the circumstances are being investigated under the direction of a senior investigating officer.
The scene in Rossfield Estate remains sealed off and a forensic examination will take place on Sunday by the Garda Technical Bureau.
The Coroner’s Office and the Office of the State Pathologist have been informed. Post-mortem examinations will also take place.
An Garda Siochana said it has appointed a family liaison officer and will continue to support those involved in conjunction with other state agencies.
Officers described it as a violent, challenging and traumatic incident which was responded to by uniform unarmed Garda first responders from Tallaght District, supported by uniform members and armed detectives and specialist armed support unit members.
An Garda Siochna also said it has put in place welfare support for all officers who attended the incident.