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.
The event had originally been scheduled for Saturday but was pushed back one day because of severe weather conditions.
"During the swim portion of Sunday's race, safety personnel provided immediate medical attention upon recognising the athletes were in need of assistance," organiser Ironman Ireland said in a statement on"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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