The Food Safety Authority of Ireland (FSAI) has warned people who may have bought the shellfish not to eat them.
The mussels were harvested from Roaringwater Bay in County Cork earlier this week and were sold across Ireland.
The FSAI said cooking the mussels will not destroy the toxin, which can cause diarrhoea, vomiting and abdominal pain.
Dr Pamela Byrne, the FSAI's chief executive, said eating the contaminated mussels could lead to diarrhetic shellfish poisoning.
"Consumers who may have already consumed this product and are feeling unwell should seek medical advice," she added.
A list of outlets that sold the affected mussels is available on the FSAI's website.
Fresh mussels have been recalled from sale in the Republic of Ireland over fears they may contain harmful levels of toxins.