Twenty-one takeaways, fishmongers and restaurants have been caught trying to con customers.
In what is the second survey to expose major seafood fraud, two unnamed suppliers have now been linked to a fish racket by food safety chiefs.
The Food Safety Authority of Ireland (FSAI) found that three-quarters of smoked fish it tested was labelled as the wrong species and that fast-food outlets were the worst offenders.
The study found cheaper pollock, hake, smelt and haddock were being used to replace the more valuable and scarce cod.
FSAI chief executive Professor Alan Reilly said the mislabelling could be a money-making racket for businesses deceiving their customers.
"If a consumer wants to buy a piece of cod, it should be a piece of cod they are buying and not some other fish," he said.
One third of the samples taken from takeaways did not match the customer's order.
The FSAI revealed that 73%, or eight out of the 11, smoked-fish samples it genetically analysed were wrongly labelled.
Ordinary white fish was more heavily tested but only 13 out of 100 were found to be labelled incorrectly.
The FSAI began checking fish and chip shops after last year's discovery by a University College Dublin scientist that a quarter of the fish sold in the city's takeaways were wrongly labelled cod.