The incident occured at Coolshine, on the Caherconlish and Herbertstown Road.
The vehicle was discovered by a passing motorist at approx 5.30am this morning, who then notified emergency services.
The road was closed to facilitate a forensic collision examination but has since re-opened.
The man’s body has been removed to the morgue at University Hospital Limerick.
Gardai in Roxboro are appealing for witnesses or anyone who may have been on that stretch of road between 6.30pm on the 10th and 5.30am on the 11th March 2016 to contact them in Roxboro Rd., Garda station on 061 214340, the Garda Confidential Line on 1800 666 111 or any Garda station.