Cork County Council said it was “deeply saddened over the tragic loss of two race participants” during the event in Youghal in south-west Ireland.
Irish broadcaster RTE said the two men, one in his mid-60s and one in his mid-40s, died in separate incidents during the 1.9km swimming portion of the competition.
The event had originally been scheduled for Saturday but was pushed back one day because of severe weather conditions.
“We share our greatest sympathies with the families and friends of the athletes and will continue to offer them our support as they go through this very difficult time.
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“We thank the safety personnel and first responders who worked quickly to provide the athletes with medical assistance.”
Both men were pronounced dead at the scene, local media reported.
They were competing in a so-called half Ironman event, which also features a 90km bike course and a 21.1km run.