Books of condolence are open following the deaths of five young Irish students and one young Irish-American in Berkeley, California.

The national flag was flown at half-mast on all State buildings as a mark of respect for those killed in the incident, which happened in the early hours of yesterday morning.

Two other students are said to be in a critical condition in hospital while five others are described as stable.

The Mayor of Galway opened a book of condolence, while University College Dublin has opened an online book of condolence.

The Lord Mayor of Cork, Chris O'Leary, opened a book of condolence in The Atrium, City Hall.

Mr O'Leary is to visit the accident site in Berkeley in the coming days as part of a planned visit to San Francisco.

He said the people of Cork were deeply upset by the tragedy. The book of condolence will be available to sign during office hours for the next two weeks.

A book of condolence will be opened at the Mansion House in Dublin from 10am tomorrow.

A book of condolence was opened at St Patrick's Cathedral in Dublin this afternoon and a spokesperson said it will remain open for the coming days.

People continue to arrive to place tributes at the scene of the #Berkeley balcony collapse. pic.twitter.com/y7w32L1DzF — Caitriona Perry (@CaitrionaPerry) June 17, 2015

The Mayor of Galway said yesterday's tragedy in Berkeley had stopped people in their tracks and led to a huge sense of sadness.

Frank Fahy was speaking after he opened a book of condolence at Galway City Hall for the victims of the balcony collapse.

He said there were 25,000 students in Galway and the city had a special bond with those in third level education as a result.

The dead have been named as Ashley Donohoe, 22, from Rohnert Park, California; Olivia Burke, 21; Eimear Walsh, 21; Eoghan Culligan, 21; Niccolai Schuster, 21; and Lorcan Miller, 21; all with addresses in Ireland.

The students were studying at UCD, DIT and the Institute of Art Design and Technology in Dún Laoghaire.

The US ambassador to Ireland has been in telephone contact with President Michael D Higgins to express his sympathy to him and to the families of those killed and injured.

Speaking in Rome earlier, where he is on an official visit, the President said he has been told that the US authorities would lend all assistance they could to the families.

The Church of Our Lady of Foxrock held a vigil in the aftermath of the tragedy last night as two of the students, Ms Walsh and Ms Burke, were from the area.

Parish priest Frank Herron, who knows Ms Walsh's family, said the community was deeply shocked and saddened by what had happened.

He said: "The tragedy is that they were heading out full of life and full of the joys of summer.

"They had the whole summer to look forward to and it’s the last thing anybody would be expecting that something like this would happen," he said.

"I was talking to one of the families whom I know well and they of course are distraught,” he added.

Arthur Godsil, retired headmaster at St Andrew's College in Booterstown, paid tribute to past pupil Lorcán Miller.

Speaking on RTÉ's Morning Ireland, he said Mr Miller was "a sportsman and he was an academic but above all he was an exceptionally lovely young man".

"He was such a personality. Looking back over his career in St Andrew's he would have given so much more back to the school than he took from it," Mr Godsil said.

"He was an engaging and an exceptional young man."

Mr Godsil said he had left a "rich tapestry" in terms of the legacy he had left.

The current principal has said counselling will be available for everyone affected at the school.

Flags flying at half mast at Loreto College Foxrock, where Olivia Burke and Eimear Walsh went to school #Berkeley pic.twitter.com/TuCHrWSftg — Glenn Mason (@glennjmason) June 17, 2015

Similarly, the flag at St Mary's College in Rathmines, is at half mast, as a mark of respect to former students Niccolai Schuster and Eoghan Culligan, who left the school in 2012.

Support services set up in UCD

Support and counselling services have been set up in UCD where three of the victims, Mr Miller, Mr Schuster and Ms Walsh, were students.

Our deepest sympathies to the families and friends of those who lost their lives or were injured in the tragic event in Berkeley, California — UCD: Uni College Dub (@ucddublin) June 16, 2015

UCD Students' Union president Marcus O'Halloran, said: "We cannot imagine what the people involved, the loved ones and bereaved are going through. It's something truly horrible.

"I would just like to reiterate to all the students at home and abroad that UCD students are here for them.

"We have a number of services. We have a welfare services and advisory service. We have a 24-hour hot line here at the moment."

College president Professor Andrew Deeks said it was an absolute tragedy for the UCD community.

Speaking to RTÉ's Morning Ireland, Prof Deeks said as a parent himself he "can't begin to think of the shock" the families are in, "the grief they are going through".

Mr Deeks said UCD, which has an office in San Francisco, has been extending all possible assistance to those effected.

He said the university's senior chaplain is flying out to the US.

Mr Deeks said while he awaited the outcome of the investigation into the balcony collapse, speaking as a civil engineer himself he knew that all balconies should be designed to take the maximum number of people that can fit on them, otherwise warning signs should be erected.

UCD is to hold an interfaith service on Friday at the Church of Our Lady Seat of Wisdom in Belfield.

More than 8,000 people have left messages on the university's online book of condolence.

Two private remembrance masses were held at St Mary's College in Rathmines and Loreto College Foxrock in Dublin this evening.

The Principal of Loreto Foxrock Bernadette Prendiville said the mass was for past and present pupils as well as their parents.

Ms Prendiville added it was an opportunity for people to gather together and grieve together.

The Ballyboden St Enda's GAA club has expressed "great sadness" at the death of Mr Culligan, who played football for the club since childhood, and his friends in the incident.

A statement on the club's website said: "Eoghan was very popular with his team mates and this tragic news is keenly felt by all members of our club, but especially by those players and mentors who knew him well."

The President of DIT, Professor Brian Norton, also extended sympathy on behalf of every student and staff member to the Culligan family.

Mr Culligan studied Logistics and Supply Chain Management at the college.

The president of Dún Laoghaire Institute of Art, Design & Technology extended their condolences to the friends and family of Ms Burke.

The 21-year-old was a third year student doing a degree in entrepreneurship and management at the college.

Speaking on RTÉ's Today with Sean O'Rourke, president Dr Annie Doona said Ms Burke had been working in a sushi restaurant in San Francisco and was doing very well and enjoying herself.

She added: "It's very dim and very quiet here on the campus this morning.

The IRFU said players on the Emerging Ireland team wore black armbands when they faced Uruguay in the Tbilisi Cup in Georgia tonight.

Our prayers with those killed by floods in Georgia & who died in California. Minute silence & black armbands to be worn by Emerging Ireland — Irish Rugby (@IrishRugby) June 17, 2015

Meanwhile, Irish celebrities have been expressing their shock and sadness on Twitter.

Beyond heartbreaking. Thoughts and prayers with all their family and friends x #Berkeley — Amy Huberman (@amyhuberman) June 16, 2015

Desperately sad news from the US, of those Irish students killed or injured. We've all done that J1 trip; we will all share the loss. — Dara Ó Briain (@daraobriain) June 16, 2015

Still completely unable to head my head around what happened in #Berkeley. So devastating — Niall Breslin (@nbrez) June 17, 2015

500 calls by families concerned about relatives

Minister for Foreign Affairs Charlie Flanagan has said his department received over 500 calls yesterday from families worried about relatives who are currently in San Francisco.

Speaking on RTÉ’s Morning Ireland, Mr Flanagan said the department offered practical support and assurance.

The minister said bereaved families have started arriving in San Francisco and the Irish consul was offering every assistance possible.

"Additional staff has been deployed on a temporary basis to the consulate in San Francisco, they've come from Washington, support from the consulate in Chicago.

"We have appointed a liaison person to deal with each family as they arrive. An incident centre has been established at the consulate.

"We are offering everything in terms of practical support to the bereaved families as they now arrive in San Francisco."

Mr Flanagan said there was concern regarding those students who had escaped serious injury but are deeply shocked.

He emphasised that students were not on their own, and that the Government's emergency crisis operation was offering every assistance possible.

He also paid tribute to volunteers who assisted at the scene of the tragedy.

The Minister for the Environment described the tragedy as "unprecedented" for Ireland.

Speaking in Galway this morning, Alan Kelly said there was a huge sense of loss at the deaths of the six students.

He said the events in California had put a lot of things into perspective, especially for parents.

The minister said he wanted to express his condolences to the families and friends of those who died.

Minister Alan Kelly describes events in Berkeley as unprecedented for our country - "huge sense of loss" pic.twitter.com/0LDjoR9yeL — Pat McGrath (@patmcgrath) June 17, 2015

Independent TD Mattie McGrath has called on the Taoiseach to initiate a national day of mourning.

Mr McGrath said the greater Irish community, and the entire student body, are in mourning today.

He added: "A day of mourning would create a healing space in which we as a nation could come together in solidarity with the families and friends of those who died."

Also speaking on Morning Ireland, US Ambassador to Ireland Kevin O'Malley said his heart went out the families of those who had died.

"This is just such a terrible tragedy that so many young Irish students with so much promise, entering into a summer where their hearts and minds were filled with ambitions to see my country and to meet Americans and establish relationships and its ended so tragically."

Mr O'Malley said the US Embassy in Dublin has made all its resources available to the Government to facilitate what it thinks is appropriate to do.

He added embassy staff have been in contact with both the Taoiseach and Mr Flanagan, offering sympathy and resources.

Chairman of the Oireachtas Joint Committee on Foreign Affairs and Trade Pat Breen extended sympathy to the families and friends of the victims.

He commended staff at the Department of Foreign Affairs for their quick response to the incident.

The committee is also to contact Consul General for Ireland Philip Grant to thank foreign affairs staff and volunteers for their assistance in dealing with the incident.