The scene of the crash in Clonmel, Co Tipperary on Friday evening
One man in his early 20s and three women in their teens have been confirmed dead.
It follows a crash which happened at around 7.30pm on Mountain Road in the town.
It is understood that the women had received their Leaving Cert results on Friday and were on their way to a party to celebrate.
Their bodies have been taken to South Tipperary General Hospital where post-mortem examinations will take place in the coming days.
The incident happened close to the entrance of Hillview Sports Club on Mountain Road.
The road is closed with local diversions in place, and is expected to remain closed overnight.
A forensic examination will take place at the scene of the crash later.
The car was travelling downhill and is believed to have gone out of control and overturned.
There was no other vehicle involved.
Gardaí are appealing for any witnesses to the crash to contact them at Clonmel Garda Station.
Any road users who may have camera footage, including dash-cam footage, and were travelling in the Mountain Road area between 7pm and 7.45pm are asked to make this footage available to gardaí.
In a statement, gardaí said they are aware of images of the immediate aftermath of the crash which are being shared on social media and in messaging apps.
They also said they are also aware of the presence of a private drone over the crash scene.
Gardaí have appealed to people not to share any such material out of respect for the dead and their family and friends.
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