The baby is in stable condition following the accident Sunday in Lough Swilly, a deep-water fjord in County Donegal.
Eyewitness Francis Crawford called emergency services but said events unfolded very quickly. He says a bystander jumped into the war and came back with the baby, but was too exhausted to go back for the others.
Authorities have recovered the bodies of two boys, a man, a woman and another female whose age is not known.
Police are treating the matter as an accident.