Michael Pickles (32) jumped to his death at Glen's Swell Season gig in San Jose, California, in 2010, landing just a few feet from the Dubliner.
"I remember afterwardsdrinking a lot at the time, and feeling really confused," Glen said in an interview with Time Out magazine in Sydney.
"There were 4,000 people sitting there, watching it happen," he said. "I mean 10-year-old kids... watching this guy die."
Glen and his band donated funds to cover counselling bills for those who witnessed the death.
And he revealed that his co-star in Once, Marketa Irglova (24) and Pearl Jam front man Eddie Vedder (48) helped him come to terms with the death.
Vedder had endured a similar tragedy in Denmark in 2000 when nine fans died in a concert accident.