Gardaí say this is the first year they are arresting more people for drug-driving than driving under the influence of alcohol.
It comes as the Road Safety Authority called for drivers to take care on the roads this Bank Holiday weekend.
Speaking at the RSA's media conference, Assistant Commissioner Paula Hilman said that in the first five months of 2021, gardaí arrested more than more 1,500 drivers with drugs in their system compared to slightly more than 1,400 with alcohol in their system.
It is the first time this has happened, she added.
Assistant Commissioner Hilman said this rising trend of drug-driving arrests continues from last year which saw a 61% increase with 3,500 arrests of people driving with drugs in their system.
She said cannabis and cocaine are the most common drugs detected.
Asked about the AGSI response to the announcement of drug testing of gardai, Mr Hilman said An Garda Síochána will continue to engage with staff representative associations to work out the finer details.
Newly released research also shows that around 10% of fatal accidents involving alcohol happened between the hours of 7am and 11am so drivers are being reminded they could still be unfit to drive the following morning.
Research from the RSA shows that a quarter of drivers surveyed agree that there were times when they may have been over the drink-driving limit when driving the morning after a night out.
According to the RSA, its Driver Attitudes and Behaviour Survey also shows that 9% of drivers admitted consuming alcohol before driving in the last 12 months, with the figure higher among men.
CEO of the RSA Sam Waide said he is concerned about the number of people driving the morning after a night out.
"Drink driving is drink driving no matter what time of the day or week it happens," he said. "The morning after is a real danger zone for drink driving.
"There is no hard and fast rule about when it is safe to drive the morning after if you have been drinking the previous night. But motorists should allow at least one hour per standard drink for the alcohol to clear their system."
The appeal to not drink and drive this Bank Holiday weekend is from the RSA, gardaí and the Medical Bureau of Road Safety.
Assistant Commissioner Hilman said that 47 people have died on Ireland's roads in 2021.
"We want everyone to enjoy this Bank Holiday weekend and to be safe on our roads, so I appeal to every driver to be responsible, and never, ever drink and drive."
The RSA said that provisional figures show that 14 people have died and 64 people seriously injured over June Bank Holiday weekends in the years 2016 to 2020.
It also said that information on the impact of alcohol on driving can be found on www.askaboutalcohol.ie.