He was the first supporter to die during a Euro 2016 soccer tournament that has been marred by off-pitch violence between fans since Friday's opening.
Nice's public prosecutor, Jean-Michel Pretre, said he had opened an investigation and it was not yet known what led to the 24-year-old's death, which involved a fall of seven or eight metres (around 25 feet).
"We're keeping all options open and have no favoured theory for now," Pretre said.
The incident took place at one end of Nice's beachfront Promenade des Anglais, which spans a wide bay. A police spokesman said that no trouble had been reported in the area.
Making their first appearance at a major finals for 30 years, the Northern Irish team lost their opening match on Sunday 1-0 against Poland.
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