The Republic of Ireland's Agriculture Minister, Simon Coveney, said results were positive for equine DNA in meat imported as raw material for the production of burgers at Silvercrest Meats in Ballybay, Co Monaghan.
Three samples were found to contain considerable quantities of horse meat and these had a common ingredient which came from Poland.
A statement from Mr Coveney said: "The investigation has therefore established a direct correlation between burgers in which a high level of equine DNA was detected and this raw material product.
"The Minister said he was confident that this finding leads to a firm conclusion that the raw material in question was the source of equine DNA introduced into burgers manufactured at Silvercrest."
He added that this was the only ingredient to test positive and said tests on samples from Irish food ingredients had come back negative.
The ABP Food Group welcomed the findings: "This has been a very difficult experience for all involved and has led to a significant interruption in business for Silvercrest and its customers. We are relieved that the source of the problem has been identified.
"While the company has never knowingly purchased or traded in equine product, I wish to take this opportunity to apologise for the impact this issue has caused."
Tesco and other supermarkets which had been stocking the burgers have since removed them from sale.