Seven others were injured when the car crashed into a crowd of 30 spectators at the race in County Cavan 50 miles northwest of Dublin, Sky News reported.
Those killed were identified as a 29-year-old woman from Cork and a 50-year-old male photographer from Galway, the news agency said.
The car's two occupants were among the injured, five of whom were treated at a hospital.
Police said the driver lost control after the car when over a hump in the road.
"I heard people screaming 'Red flag, red flag, stop the stage!' Then all hell broke loose basically. There were people crying, people white as ghosts," witness Matthew McMahon told Irish state broadcaster RTE.
Police Superintendent Gerry O'Brien said investigators were working to determine the cause of the crash. Organizers canceled the remainder of the 100-car race after the accident.
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