The latest victim, a 22-year-old male pedestrian who was struck by a van on the N21 Croagh to Rathkeale road in Co Limerick at 3.30am yesterday, has been named as Stephen Duggan from Feoghanagh.
He was pronounced dead at the scene and his body was taken to University Hospital Limerick for a post mortem.
The occupants of the van, a male driver and a female passenger in their late 20s, were uninjured in the collision.
Meanwhile, gardaí are investigating if the driver of a car in which a man was killed and two others injured in Co Donegal may have swerved to avoid something just before the tragedy.
John Hegarty, 21, from Listen Avenue, Buncrana, was killed instantly when the car in which he was a passenger crashed into a bollard at Tooban, outside Burnfoot, around 11.40pm late on Sunday night.
Two other young men, an 18-year-old and a 21-year-old, were also injured and rushed to Letterkenny General Hospital. Their injuries are not life-threatening, however.
The man who died was understood to be a back seat passenger in the silver Toyota Corolla involved in the smash.
The latest fatalities follow the deaths of Martin Brophy, 20, Angela Kelly, 35, and Orla Curry, 20, in a collision in Rathdowney, Co Laois, in the early hours of Sunday morning.
Garda forensic collision investigators are attempting to identify the causes of all three fatal crashes.
The five deaths bring to 10 the number of people who have lost their lives on Irish roads so far this month.
RSA spokesman Brian Farrell said that it had been an “appalling weekend” and “one of the worst weeks to date this year”.
“These tragedies serve as a reminder to us all that getting in a car and driving is the most dangerous thing we do on a daily basis,” Mr Farrell said. “Routine journeys can turn to tragedy in an instant.”
So far in 2015, 82 people have died on Irish roads, in comparison to 103 over the same period last year. In 2014, a total of 195 people died on the roads, 20 of them in the month of July.