The two-car accident happened on the N2 at Blakestown, Ardee, Co Louth at around 11.40pm on Sunday, October 25.
Gardaí [Irish police] and emergency services attended the scene.
A statement from Gardaí on Monday confirmed the road remains closed for a technical examination by Forensic Collision Investigators.
They added that local diversions are in place.
Gardaí said: ‘The occupants of one vehicle, two men (50s and late teens) and two women (50s and 20s), were removed from the scene to Our Lady of Lourdes Hospital Drogheda for treatment of serious injuries.
"An occupant of the second vehicle, a male aged in his 30s, was also taken to Our Lady of Lourdes Hospital Drogheda for treatment of injuries believed to be non-life-threatening."
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Investigating Gardaí are appealing to anyone who may have witnessed the collision to come forward.
They said: "Any road users who were traveling in the area of the N2 at Blakestown, between 11.20pm and 11.50pm, and who may have camera footage (including dash-cam) are asked to make it available to Gardaí."
Anyone with any information is asked to contact Drogheda Garda Station on 041 987 4200, the Garda Confidential Line on 1800 666 111 or any Garda station.
* This article was originally published on Extra.ie.