Politicians unite to remember dead
The normal business of the Dail was suspended in one of many marks of respect for the dead in the US balcony collapse.
Taoiseach Enda Kenny led messages of condolence to the parents, families and friends of those affected by the tragedy while a minister has been dispatched to San Francisco to lead the consular response on the ground.
"We've all been shocked by the loss of life and injured," Mr Kenny said.
"It is a terrible situation to have such a serious and sad incident to take place at the beginning of a summer's activity and opportunity for so many young people on J1 visas in the US."
A team of Government officials were on hand in Dublin Airport to support relatives travelling to the US west coast while consular staff from the Irish diplomatic corps in San Francisco met them on arrival.
Jimmy Deenihan, junior minister responsible for issues relating to the Irish diaspora, has been asked to lead the response in California and liaise with families alongside local consul general Philip Grant.
Mr Kenny said he would act as a Government presence in solidarity with the affected families and the young people who remain in San Francisco.
Flags flew at half mast at Government Buildings, at University College Dublin where three of the students studied and at the US embassy in Dublin.
Books of condolence were also opened by the university, with thousands of people signing online, while other books were being opened in the Mansion House in Dublin and in Cork and Galway.
Tanaiste and Labour Party leader Joan Burton, a former J1 student, said there are no words to describe the horror of the events at Berkeley.
"A J1 is meant to be a rite of passage, an opportunity to gain valuable life and cultural experience in a country, the US, that's so dear to all of our hearts.
"It is for a lot of people the summer of love, the summer of fun, and when you see those faces today in the snapshots on social media and the newspapers I think it brings back to everybody what those days are meant to be.
"Today though six families are heartbroken. Their children are wrenched away from them in the most dreadful of circumstances."
Micheal Martin, leader of the main opposition party Fianna Fail, said everybody in Ireland was thinking of the grieving families.
"The J1 programme is essentially a programme for young people and it brings to mind opportunity, a summer of fun, a summer of happiness, new eras beckoning, relationships and so forth," he said.
"That's why it has such a resonance."
Gerry Adams, Sinn Fein president, said: " It's easy to imagine the energy, the fun and excitement at that party before the disaster.
"It's a dreadful, stark reminder of the fragility of life, of the uncertainty of life, especially when the victims are so young, so vibrant and so full of potential."
Books of condolence were also opened at St Patrick's Cathedral in Dublin and University College Cork.
Foreign Affairs Minister Charlie Flanagan said concerns were also turning to the impact the trauma will have on those who survived the accident and those who witnessed it.
"We are also conscious that many Irish students were not physically injured, but were left deeply shocked and saddened by the loss of friends and classmates in this terrible accident," he said.
"The consulate has worked with local authorities in Berkeley to set up an incident centre in Berkeley, where grief counsellors will be on site and people will also have facilities to make phone calls home."
Extra diplomatic staff have been flown into San Francisco to support families while local authorities in Berkeley and Irish community organisations in the Bay area have been working together to provide transport and accommodation to those who need it.