Test results have confirmed that a raw material ingredient at Rangeland Foods, Co Monaghan, was 75% horse meat.
Minister for Agriculture Simon Coveney has asked gardaí to join the investigation into the presence of horse DNA in beef products at Irish factories.
The request follows the latest test results and the facts previously uncovered at Silvercrest Foods.
Mr Coveney said that Rangeland Foods last Thursday notified his department of its use of Polish meat ingredients in some burger products.
The company, which produces beef burgers, has suspended production.
Rangeland Foods in Castleblayney said the consignment was received in early January but did not go into production.
It said production has been temporarily suspended and the test results were immediately reported to the Department of Agriculture.
In a statement, it said that 90% of Rangeland’s beef usage is of Irish origin.
Mr Coveney said his department has had inspectors in the plant since 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.
The minister has asked both gardaí and the Agriculture Department’s Special Investigation Unit to join the investigation.
Speaking on RTE's Prime Time, Mr Coveney said he was not surprised that a raw material at the plant contained horse DNA.
He said the product, which was sourced from Poland, was the same ingredient that was being supplied to the Silvercrest plant.
Minister Coveney said that Silvercrest and Rangelands were the only burger manufacturers using this product.
He said he had invited Polish investigators to Ireland to see the evidence that has been gathered.
He said there was proof that a lot of product same to Ireland from Poland as there is documentation showing orders, invoices, payments and trucks leaving Poland full of product and coming to Ireland.
Polish authorities have said that tests have so far proved negative for horse DNA.
Meanwhile, the UK's Food Standards Agency has said it tested a quantity of frozen meat in a cold store at Freeza Meats in Northern Ireland, which "is potentially linked to the Silvercrest factory in the Republic of Ireland".
A statement on the agency's website said of the 12 samples that have been tested, two have come back positive for horse meat at around 80%.
"The investigation into the traceability of these raw materials and their source is under way. As this meat was detained, it has not entered the food chain".
Reacting to the test results, IFA President John Bryan said this is further evidence that only Irish raw material must be used in the manufacture of Irish beef burgers.
He said it is clear that stricter controls must apply to the secondary processing industry, particularly where imported products are being used.
Irish Cattle and Sheep Farmers’ Association President Gabriel Gilmartin said the news was "a deeply worrying development and has left farmers furious.
"It suggests that the minister will now have to move quickly to investigate all meat processing plants."
Mr Gilmartin also called on all processors who have any grounds for concern regarding suspect ingredients to immediately come forward.
Supermac's said all the beef in its burgers is 100% Irish and fully traceable back to the farm.
In a statement, the fast-food chain said the product in question is imported product and "bears no relevance to Supermac's meat products which are 100% Irish".