Meat used for production of burgers at Silvercrest Foods in Ballybay, County Monaghan, came in from Poland and was not of Irish origin, the Republic of Ireland's Agriculture Minister, Simon Coveney, said this weekend.
In tests, 3 samples which were found to contain considerable quantities of horse meat all came from Poland.
"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," said minister Coveney.
"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 tests on samples from Irish food ingredients had come back negative.
The announcement follows the discovery last month of a burger supplied for the Tesco value range where 29% of the meat content was not beef but horse.
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."