The fifth victim was rushed to the nearby Wilcox Memorial Hospital where he later died, officials confirmed.
The group was believed to have been part of a tour operated by SkyDive Kauai, county firefighters said. The company offers tours from Port Allen.
The cause of the crash is now under investigation.
A spokesperson for the department has said that consular assistance is being provided to the family of the Irish man who died.
"By the time I got there, the plane had been burned pretty bad", Kauai Fire Chief Robert Westerman said. A small plane with two people aboard reportedly crashed in the waters off Makaha later Monday.
Cisco Campos, a retired Air Force aircraft mechanic, said he was fishing with his wife Monday morning when he saw the Skydive Kauai plane take off and noticed that the engine sounded odd. It's unclear what led to the crash.