Scientific tests on beef products sold in Tesco, Lidl, Aldi, Iceland and Dunnes Stores uncovered low levels of the animal's DNA.
A total of 27 products were tested by the Food Safety Authority of Ireland (FSAI), with ten of them containing horse DNA and 23 pig DNA.
Horse meat accounted for approximately 29% of the meat content in one sample from Tesco, the UK's biggest food retailer.
Beefurger products which tested positive for horse DNA were produced by Liffey Meats and Silvercrest Foods in Ireland and one UK plant, Dalepak Hambleton.
Silvercrest said it was pulling products from sale and replacing them with new lines.
Some 31 beef meal products such as cottage pie, beef curry pie and lasagne were tested, with 21 found to be positive for pig DNA. All tested negative for horse meat.
The DNA tests found horse in the following products: Tesco Everyday Value Beef Burgers 29.1%; Tesco Beef Quarter Pounders 0.1%; Oakhurst Beef Burgers in Aldi 0.3%; Moordale Quarter Pounders in Lidl 0.1%; Flamehouse Chargrilled Quarter Pounders in Dunnes Stores 0.1%; two varieties of Iceland Quarter Pounders 0.1%.
Even lower levels were recorded in Moordale Beef Burgers in Lidl and St Bernard Beef Burgers in Dunnes Stores.
The retailers have told food safety chiefs they are removing all implicated products from their shelves.
Professor Alan Reilly, chief executive of the FSAI, said there was no health risk but also no reasonable explanation for horse meat to be found.
He said: "The products we have identified as containing horse DNA and/or pig DNA do not pose any food safety risk and consumers should not be worried."
"Whilst there is a plausible explanation for the presence of pig DNA in these products due to the fact that meat from different animals is processed in the same meat plants, there is no clear explanation at this time for the presence of horse DNA in products emanating from meat plants that do not use horse meat in their production process.
"It is not in our culture to eat horse meat and therefore, we do not expect to find it in a burger."
Prof Reilly said traces of other meats would be unacceptable for people who may not eat certain food on religious grounds.
He added: "Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."
Tim Smith, group technical director at Tesco, said: "Today we were informed that the FSAI has found that a number of beef products they have recently tested contained horse DNA.
"These included two frozen beef burger products sold by Tesco in both the UK and Ireland. Products sold at other retailers were also discovered to contain horse DNA.
"We immediately withdrew from sale all products from the supplier in question. We are working with the authorities in Ireland and the UK, and with the supplier concerned, to urgently understand how this has happened and how to ensure it does not happen again.
"We will not take any products from this site until the conclusion and satisfactory resolution of an investigation.
"We understand that many of our customers will be concerned by this news, and we apologise sincerely for any distress. Our customer service team is standing by to answer any questions customers may have."
Aldi said they were conducting their own investigation. "We have sought information from one supplier, Silvercrest, which is dealing directly with the FSAI on the issue that has been raised," the company said.
In a statement, Lidl said it has taken the decision to remove all implicated products from sale pending a full investigation.
"A refund will be provided to customers who wish to return affected products," said a spokesman.
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