Gardaí believe a young child and its parents were among four people who died following a traffic collision on the M6 in Co Galway on Thursday night.
The driver of another car was also killed in the crash.
The deaths bring to 92 the number of people who have lost their lives on Irish roads this year.
Detectives have spent most of the day trying to piece together the exact circumstances that led to the tragedy.
The crash happened on a stretch of the main Dublin to Galway motorway, a short distance from Ballinasloe.
Shortly after 7.30pm yesterday, a car was seen driving towards oncoming traffic on the westbound carriage way of the M6.
It is thought the single-occupant vehicle travelled the wrong way down a slip road and onto the dual carriageway.
Soon afterwards, it collided head-on with a car carrying three people.
The occupants of both cars were pronounced dead at the scene.
Their bodies were taken to Portiuncula University Hospital, where post mortem examinations are due to take place.
Another motorist, travelling in a third car, was taken to hospital after sustaining non-life-threatening injuries.
Garda collision investigators completed a detailed examination of the site this afternoon.
After that process, the cars involved in the crash were taken from the scene, for further analysis.
Efforts to formally identify all the victims and contact their relatives are continuing.
But it is understood that the extent of the injuries sustained by some of the deceased has complicated this task.
Gardaí are hoping that post-mortem examination results will clarify the situation.
The M6 reopened to traffic this evening after a section of the motorway was closed in both directions for much of the day.
M6 now reopen in both directions, following last evening's collision, which claimed the lives of four people.
— Pat McGrath (@patmcgrath) August 20, 2021
Cars removed from scene this evening . More on @rtenews pic.twitter.com/CN1AuWyMNw
Gardaí are also appealing to any witnesses or any road users who may have camera footage (including dash-cam) and were travelling on the M6 at Poolboy, Ballinasloe, at the time, to contact Ballinasloe Garda Station on 090 9631890, the Garda Confidential Line on 1800 666 111, or any garda station.
Meanwhile, a woman was injured in a collision on the M6 near Athenry in Co Galway this evening.
The crash involving two vehicles happened eastbound on the motorway at Junction 18 Rathmorrissy at around 4.30pm.
The woman was taken to University Hospital Galway to be treated for injuries which gardaí said were not life-threatening.
