At least eight others have been injured in the incident which happened in the university city of Berkeley in California, in the early hours of this morning.
Ireland's Foreign Affairs Minister Charlie Flanagan confirmed the tragedy involved Irish students who were in the US working for the summer.
Archbishop Eamon Martin, leader of the Catholic Church in Ireland, said they were "tragic deaths" and that he was "shocked and deeply saddened".
He added: "This devastating news will be felt by Irish people everywhere, and particularly by those who have family and friends abroad.
"I pray that the eight injured students will fully recover from this terrible accident.
"The death of a child or young person is the worst news that a parent can receive. At this profoundly painful time I pray for those who have died, and ask also for prayerful support for their grieving families, fellow students and loved ones."
All of those who were killed or injured are believed to be aged between 20 and 22.
Police in California have not given any details on how the balcony collapsed although officers are investigating how many people were on it when it collapsed.
Social media images appeared to show the balcony broken off from the building and in pieces on other balconies and the street.
Mr Flanagan said: "It's an appalling tragedy, an appalling loss of life for young people whose hopes and dreams of the future have suddenly and without notice been shattered.
"My heart goes out to the families and loved ones of the deceased and those who have been injured in this appalling accident."