O'Neill led the tributes to Mr Darren Rodgers who, according to officials, fell on his head on a stone beach after scaling a seafront railing in Nice.
The coach said the incident put his side's 1-0 defeat by Poland in their opening game on Sunday into perspective.
Fellow supporters created a makeshift shrine to honour Mr Rodgers, 24, placing a host of green shirts and scarves next to the railings.
"It was devastating news when we got up this morning having travelled back last night after the game to find out the news about young Darren Rodgers," said O'Neill who hails from Ballymena, the same town as the victim.
"It puts into perspective everything for the players. I know they were devastated to hear the news ... we can only pass on our condolences and our sympathies to his family, friends and everyone connected to the boy.
"Hopefully come Thursday (when the Irish play Ukraine in Lyon) the team and supporters will show their togetherness for Darren and his family and I know it will be a fitting tribute," added O'Neill.
Mr Rodgers was the first supporter to die during the tournament which has been marred by off-pitch violence between fans since Friday's opening.
(Writing by Ian Chadband; Editing by Tony Jimenez)
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