A driver has been caught doing 152 kilometres per hour on the M7 in Kildare, while another was found doing almost 100 on Main Street in Oola in Limerick.
Gardai say a total of 454 drivers have been detected breaking the speed limit.
Almost 90,000 speed checks have been carried out so far as part of National Slow Down Day.
The force has provided a list of examples where it says drivers were putting themselves "at risk."
The Garda operation runs until 7am tomorrow.