The Department of Agriculture confirmed damaging findings of 75% equine DNA in raw ingredient at Rangeland Foods in Co Monaghan.
Just days after the ABP Food Group lost an estimated 45 million euro (£38m) in contracts over the deepening scandal, Irish police have been called in to aid inquiries.
A special investigation unit from the department has been tasked to get to the bottom of the controversy.
In a statement the department said production has been suspended at Rangeland, a frozen burger supplier established in 1892 with a turnover of 18m euro (£15m) and about 80 staff.
"The company has indicated that none of this product has entered the food chain," the department said.
Inquiries into whether Polish labelled product has been used in other meat processing plants in Ireland are ongoing.
Rangeland called in authorities last Thursday amid suspicions that Polish sourced meat may contain horse.
An Irish-based trader had imported the meat, the department said.
"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 department said.
The Rangeland results were released on the eve of a briefing Irish Agriculture Minister Simon Coveney is to give politicians in Dublin at a parliamentary committee on the wider horse meat controversy.
The meeting was arranged after the Silvercrest processing plant in Monaghan, part of the ABP group, was found to have supplied products contaminated with horse.
Irish authorities have been liaising with Polish officials over the source of the contamination.
"The investigation has shown that all implicated raw material ingredient is labelled as Polish product," the department said.
Aside from reputational damage to Ireland's 10 billion euro (£8.5bn) agri-food industry, the ABP Food Group, owned by Larry Goodman, has lost contracts with Tesco, Aldi, the Co-Operative Group and Burger King over the fiasco.
Mr Coveney will join Professor Alan Reilly, whose research at the Food Safety Authority of Ireland (FSAI) first exposed the contamination of processed beef burgers.
Controversy and concerns about traceability of food widened at the weekend when a company which supplied halal food found to contain traces of pork DNA was named by food distributor 3663 as McColgan Quality Foods Limited, a Northern Ireland-based company.