The deaths of six Irish students in the balcony collapse in Berkeley, California, marked a "horrific day" for those who had lost loved ones, Minister for Foreign Affairs Charlie Flanagan said.
Six students died and nine were injured in the incident. Their identities were still being confirmed on Tuesday evening.
Speaking to reporters in Dublin, Mr Flanagan said he understood all the dead and injured were Irish J1 students.
"Today is an horrific day for those who have lost loved ones in tragic circumstances in San Francisco and for all those affected. In my Department and in our Consulate in San Francisco, the students and their families have been at the centre of our thoughts and actions today."
Mr Flanagan said the Irish consulate in San Francisco and the Department’s Emergency Consular Response Team in Dublin continued to work with the local authorities to identify and make contact with the families of all the Irish students involved the “tragic incident”.
“The identities of the deceased Irish citizens are in the process of being officially confirmed. Our thoughts and heartfelt sympathies are with the families and friends of the deceased at this most difficult time,” Mr Flanagan said.
“We are also working with the authorities across several hospitals to confirm the extent of the injuries suffered by our citizens involved, and to relay this information to families as soon as possible.
Ireland’s consul general in San Francisco was in close contact with Irish community organisations on the ground, and Mr Flanagan said he welcomed their offers of assistance and expression of solidarity at this difficult time.
The consular emergency response phone line had received an “unprecedented” volume of calls on Tuesday afternoon.
“This demonstrates that this is an incident that has touched many Irish families – not just those whose sons and daughters were actually there, but all families who have a loved one travelling this summer.”
He urged any students in the San Francisco area who had not yet been in touch with their families in Ireland to please contact them.
Tánaiste Joan Burton said her heart went out to the families and that hearing such news about a loved one far away would be "any parent's worst nightmare".
“I will be monitoring the situation with regard to those who have been hurt in the incident and I sincerely hope that those being treated for serious injuries will make a recovery.”
President Michael D Higgins, who was on an official visit to the EXPO 2015 exhibition in Milan, said his heart went out to the families and loved ones of all those involved.
Martin McGuinness, Northern Ireland's deputy first minister, also expressed his sympathies.
“Shocking news of the deaths of several young Irish people in a tragic accident in California. My heart and sympathy goes out to their families,” he said.
Archbishop Eamon Martin said the death of a child or young person was the worst news that a parent could 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."
Laura Harmon, President of the Union of Students in Ireland, said: "The thoughts and sympathies of the whole student movement in Ireland are today with the families and friends of those young people caught up in this terrible event. We are anxiously awaiting further information from the authorities.''
The Department of Foreign Affairs emergency helpline is 01 418 0200.
