A child was among those killed in a multi-car crash near Ballinasloe in Co Galway.
It is understood a family of three were among the four people who died in the incident at around 7:40pm on Thursday.
It is believed the three-vehicle crash happened after one of the cars was travelling in the wrong direction along the M6 motorway.
A family of three - a mother, father and child - were thought to be in one of the vehicles.
They are not believed to be from the local area.
The occupant of one of the other cars also died - while the driver of the third was taken to hospital with non life-threatening injuries.
The bodies of those killed were taken to Portiuncula Hospital, where post-mortem exams will take place.
The road was closed earlier between junctions 14 and 16 to allow for forensic examinations to take place.
Anyone with information about the crash - including anybody who may have camera footage from the area - is asked to contact Ballinasloe Garda station on 090-963-1890, the Garda Confidential Line at 1800-666-111 or any Garda station.