uthorities in California confirmed that seven others remain in hospital.
The collapse happened during a 21st birthday party shortly before 01:00 local time (08:00 GMT) on Tuesday in the city of Berkeley.
The students are believed to have been living temporarily in the US as part of a work exchange programme.
Around 700 Irish students are currently living in the San Francisco Bay Area, said Philip Grant, the Consul General of Ireland to the Western United States.
"We're still in an emergency response mode," he said. "It's a formative experience, and to have this happen ... has left us all frozen in shock and disbelief".
The Library Gardens apartment complex, completed in 2007, had wood-frame construction, and the balcony had no additional support, making it more vulnerable to dry rot and weathering, Grace Kang, a structural engineer and spokeswoman for Pacific Earthquake
Engineering Research Center at Berkeley, told the AP.
Inspectors also planned to see if the balcony was too packed or overloaded.
Whatever its cause, Berkeley Mayor Tom Bates vowed to “get to the bottom” of it, calling the collapse a “really shocking set of events.”
Pictures show the balcony had completely detached, coming to rest on the balcony for the floor below. The building, which has retail space on the ground floor and apartments above, is located just blocks from University of California Berkeley.
