PRESIDENT Michael D Higgins led messages of sympathy and condolences to the families of the dead and injured in the tragedy.
The President was informed of the incident during an official visit to Italy and described the deaths as a "terrible loss of life" of young people.
"My heart goes out to the families and loved ones of all those involved," said President Higgins.
"I have been informed of the consular assistance being provided to assist all of the families involved and I have asked to be kept informed as further details emerge."
In the Dail, Taoiseach Enda Kenny told a hushed chamber: "My heart breaks for the parents who have lost their children this morning."
Mr Kenny said his thoughts were also with other parents waiting fretfully for news from children who may be currently visiting California. He said he had been contacted by the US Ambassador to Ireland Kevin O'Malley to relay the sympathies of the American people.
The Taoiseach said Ambassador O'Malley had recently welcomed a number of Irish students preparing to travel to the USA on J1 visa. Mr Kenny said it was doubly tragic that such an incident could occur "at the start of a summer adventure" .
Foreign Affairs Minister Charlie Flanagan, who broke the news of the deaths, said: "This is a very challenging and difficult time for the families, for parents of those directly involved. It has been an appalling tragedy, an appalling loss of life.
"Young people with hopes and dreams for the future have suddenly been shattered."
He said the department's consular services, headed by top official Philip Grant, is assisting those affected and is at the scene of the tragedy.
Fianna Fail leader Micheal Martin said his thoughts and prayers were with the bereaved families and injured people.
Irish Consul General in San Francisco Philip Grant was in Berkeley.
"We have thousands of Irish students that come to the United States every summer on an exchange programme. They work here and gain valuable experience.
"They get to experience America and it is a wonderful opportunity.
"For many it's a formative experience and to have this happen at the start of this season, is something that has left us all frozen in shock and disbelief," Mr Grant said.
"We have been overwhelmed by the outpouring of support that we have received from all the communities here, in particular the Irish community in the Bay area. We are a very close tight-knit group.
"We get around 700 students in the Bay area at the moment. There are very few of them who wouldn't know someone at that party, someone who was unfortunately was injured, or unbelievably lost their lives in such a tragic situation last night," he said.
Tanaiste Joan Burton said, "hearing news like this about a loved one far away would be any parent's worst nightmare".
She added that everyone in Ireland was hoping the injured students would make a full recovery.
Student leaders said the deaths and injuries had left colleagues devastated.
"The thoughts and sympathies of the whole student movement in Ireland are with the families and friends of those young people caught up in this terrible event," Laura Harmon, USI President, said.
"We are anxiously awaiting further information from the authorities.''
The Catholic Primate Archbishop Eamon Martin said news of the tragedy was being felt by Irish people everywhere "particularly by those who have family and friends abroad".
"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," he said.
The Lord Mayor of Dublin, Christy Burke, will open a Book of Condolence for the victims.
The public will be able to sign the Book of Condolence at the Mansion House on Dawson St on Thursday and Friday from 10am to 4pm.