One third of vehicles that property was stolen from in residential areas were left unlocked, according to figures released by An Garda Síochána.
The figures from the Garda Analysis Service show that the most common location for thefts is when a car is parked in a driveway.
Speaking on RTÉ's News at One, Sergeant Kelvin Courtney said people need to "park smart" and remove valuables from their vehicles.
He encouraged people to park securely, put on their alarm, park in well-lit areas and not to leave property on display.
In total property was stolen from 8,571 vehicles at residential locations between 2015 and 2017. Of those, 2,824 were left unlocked.
Half of the thefts occurred between midnight and 7am, with an average of €340,000 stolen from vehicles each year.
The type of property stolen included cash, laptops, tools, sunglasses, jewellery and sports equipment.
Sgt Courtney added that in cases where it is impractical to remove the contents of a vehicle, such as tools from a work van, additional security measures should be fitted.