The San Francisco Chronicle and the Los Angeles Times reported that the death toll had risen to six, but that could not be immediately confirmed by Reuters.
The Irish victims had been celebrating a friend's 21st birthday when the tragedy occurred, according to the Chronicle.
Pictures from the scene showed that the fourth-floor balcony gave way and pancaked onto the third-story balcony just beneath it at the Library Gardens apartment complex, about two blocks from the UC Berkeley campus.
Berkeley Police spokeswoman Jennifer Coats confirmed that at least five people were killed but did not give the nationality of those involved. The survivors' injuries were "very serious and potentially life-threatening," she said.
Irish Foreign Minister Charlie Flanagan said from Dublin that the five confirmed fatalities were all young Irish citizens and that police had indicated they did not believe individuals of other nationalities were among the dead.
"My understanding is that four people were declared dead at the scene, one later died in hospital. Up to eight or nine others have been taken to hospitals. Those involved are believed to be Irish students for the most part," Flanagan told national broadcaster RTE.