The standards watchdog has handed out warnings to the operators of 30 taxi meters and 270 diesel and petrol pumps.
It is after they failed inspections, according to the National Standards Authority's annual report.
A total of 14,000 items were inspected in 2017.
Geraldine Larkin, Chief Executive of the NSAI, says there was one incident where someone was prosecuted.
"If we suspect an instrument may have been tampered with purposely, we can prosecute the trader in question," said Ms Larkin.
"This year, we took a significant prosecution in Donegal against a fish processing plant where a weighing scales had been deliberately tampered with.
"Following from that, a court prosecution awarded fines and costs of the tune of €45,000."