Government officials found 75% horse DNA in raw ingredients at Rangeland Foods in Co Monaghan and brought in police.
Rangeland bills itself as Ireland's number one producer of beef- burgers to the food service industry with customers in the UK, Greece, Belgium, Sweden, Spain, Holland, Cyprus and Gibraltar.
Ireland's Agriculture Department said production has been suspended at the second frozen burger supplier which employs 80 staff, but stressed none of the product had entered the food chain.
Rangeland called in authorities last Thursday amid suspicions Polish sourced meat may contain horse.
An Irish-based trader had imported the meat, the department said, adding: "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."