Irish authorities suspended production at a meat processing plant on Friday after investigators found it was selling horsemeat labelled in the Czech Republic as beef, the agriculture ministry said.
B&F Meats, a small company licenced to debone beef and horsemeat in Carrick-on-Suir in County Tipperary, was found to be sending horsemeat to a customer in the Czech Republic, it said in a statement.
The label in the Czech language refers to beef, it added.
"I am seriously concerned about this development and the Gardai (Irish police) have been fully appraised of this development and are working closely with my Department," agriculture minister Simon Coveney said.
"The issue here is one of mislabelling and that will be the focus of the investigation," he added.
The ministry said the horsemeat was sent to the Czech Republic through a British-based trader but did not reveal details.
A spokesperson told AFP they have been in contact with Britain's Food Standards Authority since the discovery.
Ireland has been at the heart of Europe's horsemeat scandal after tests by the Irish Food Safety Authority last month discovered equine content in a number of beef products.
Fifty 50 additional food samples will be checked for horse DNA during March in Ireland, the ministry said.
Coveney, who is acting chairman of the EU Council of Agriculture and Fisheries as Dublin holds the rotating EU presidency, has arranged a special debate on the issue next Monday in Brussels.