The accident happened on the M8 just south of Mitchelstown, Co Cork, at around 11.15am.
It is believed the man had pulled his car onto the hard shoulder of the southbound lane and got out of the vehicle before he was struck.
Gardaí said the man’s age was not known, but it is thought that he lived in the North Cork area.
Supt Mick Comyns, who is leading the investigation, yesterday said gardaí were trying to trace the man’s relatives.
He said gardaí had spoken to a number of witnesses, but appealed to anybody who may not yet have come forward to contact Mitchelstown Garda Station at 025 84833.
Gardaí closed the southbound section of the road between junctions 13 and 14 while forensic investigators examined the area.
Meanwhile, gardaí have renewed their appeal for information following a fatal road traffic collision in Dún Laoghaire, Co Dublin on Tuesday, Jun 12.
A man aged in his 30s died in the accident at Kill Avenue, at about 2.15pm.
Gardaí are asking anyone with information, or anyone who was in the area between 2pm and 3pm that day to contact Dún Laoghaire Garda Station at 01 6665000, or the Garda Confidential Line at 1800 666 111.
In Co Galway, a teacher who last week celebrated his 25th birthday lost his life in a single-vehicle accident near his home at around 11.30pm on Monday night.
James O’Toole from Cappagh, Dunmore in Co Galway had had triple cause for celebration over the past week.
Aside from his birthday on Monday of last week, he had just discovered his contract as a woodwork teacher in St Colman’s College in Claremorris, Co Mayo, had been renewed.
And at the weekend, the talented Gaelic footballer scored two points as he helped Dunmore McHales to victory over Monivea/ Abbey in the Galway intermediate championship.
However, he lost his life when his car collided with a ditch as he made his way home from watching the Euro 2012 game between the Republic of Ireland and Italy on Monday night.
The death of the popular young man has shocked the north Co Galway community.
Tom Ryder, Dunmore McHales chairman, said all at the club were stunned by the news.
“We are all desperately sad. We are devastated and very upset. James was such a fantastic clubman and he was so excited about our win at the weekend.”
Mr O’Toole is survived by his father Seamus, his mother Sally, his brothers Ciaran, Stephen, and John, and his two sisters, Paula and Sarah.