Police found debris on the street below after an emergency call at 01:00 local time on June 16.
Officers arriving at the scene found that the balcony on the fourth floor of an apartment building on Kittredge Street, near the University of California, Berkeley, had collapsed.
The students are believed to have been holding a birthday party at the time, Irish officials said, but the cause of collapse is unknown.
They were believed to be in the US on temporary worker visas.
Photos taken at the scene appear to show a balcony on the fourth floor of the building fallen to the balcony on the floor below.
Berkeley Police Department spokeswoman Jennifer Coats said that many of those hurt have life-threatening injuries.
Officer Jennifer Coats said that the police were still investigating and that she did not have any information on what the people were doing on the balcony at the time.
Four of the victims died at the scene and another died in hospital, said Irish Foreign Minister Charlie Flanagan.
“It is with great sadness that I confirm that a number of young Irish citizens have lost their lives,” Charlie Flanagan said, “while a number of others have been seriously injured following the collapse of a balcony in Berkeley.”
He said another eight or nine people who were injured in the incident were Irish students as well.
The identities of the victims are not yet being released, pending family notification.
Irish President Michael Higgins said that he had “heard with the greatest sadness of the terrible loss of life of young Irish people and the critical injury of others in Berkeley, California today”.
He said his heart goes out to the families and their loved ones.
The Irish consul general in San Francisco is helping those affected and there is an Irish helpline (+353 1 418 0200).
The building has apartments in the upper floors and shops on the ground floor.
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