Páraic Casey, 45, was less than 1km from the French coast and more than 16 hours into the crossing when he took ill in the early hours of Sunday morning.
He could not be resuscitated despite the immediate efforts of safety personnel.
The swimmer from Passage West in Co Cork has begun the open sea crossing just after 9am on Saturday to raise funds for St Vincent de Paul and St Patrick's Marymount Hospice.
According to The Irish Times, he had passed a standard medical examination, had completed the necessaary training and was wearing a wetsuit for the swim.
Casey is survived by his wife Riana, who described him as “amazing”.