Horsemeat has been found in beef at another meat processor in the Republic of Ireland.
Rangeland Foods in Castleblayney, County Monaghan, has temporarily suspended production after beef from Poland tested positive for horse DNA.
Ireland's Department of Agriculture said it had received a test result confirming 75% horse DNA in a raw material ingredient at the firm.
The company said the consignment did not go into production.
It said the test results on the meat, which it took delivery of in early January, were immediately reported to the Department of Agriculture.
The company said that 90% of the beef it uses is of Irish origin.
Established in 1982, the company has more than 80 employees.
In a statement agriculture minister Simon Coveney said he had asked the police to join the investigation into how horsemeat got into beef products at Irish factories.
"The company has indicated that none of this product has entered the food chain," he said.
"The department has had inspectors in the plant since last Friday. The investigation is focusing on the full supply chain including the meat trader concerned and others who facilitated the purchase of the product and its transfer to users in Ireland."
Last month Irish food inspectors found horse DNA in some beefburgers being sold in UK and Irish supermarkets.
The meat came from two processing plants in Ireland, Liffey Meats and Silvercrest Foods, and the Dalepak Hambleton plant in Yorkshire.
The burgers had been on sale in Tesco and Iceland in the UK and the Republic of Ireland, where they were also on sale in Dunnes Stores, Lidl and Aldi.
The officials said there was no risk to human health and the burgers had been removed.
Since then Tesco, Aldi, the Co-Op and Burger King have dropped Silvercrest as a supplier.
On Sunday, it emerged that a food producer in Northern Ireland had supplied halal food containing traces of pork DNA to prisons.
McColgan's Quality Foods Limited was the source of "the very small number of halal savoury beef pastry products," said food distributor 3663.
The County Tyrone company said it was co-operating with The Food Standards Agency.
3663 carried out tests on five products after suspecting the halal products may have contained horsemeat.
A 3663 company spokeswoman said it was "shocked" to find pork DNA traces.
She added that the affected products had been withdrawn from supply and were only distributed to prisons.
Under Islamic law, Muslims are required to eat halal food - and eating pork is strictly forbidden.