President Michael D Higgins has said the Las Vegas gun attack was appalling and sent shock around the world.
No Irish people are believed to have been killed or injured in the indiscriminate shooting that left scores dead at the Route 91 Harvest country music festival.
It is estimated there are at least 200 Irish people in Las Vegas at the moment. Tour operators say they will arrange for tourists to cut short their trip to the city if requested.
Mr Higgins said: "The appalling massacre of music lovers of different generations, at a public event in Las Vegas will have shocked people all over the world.
"On behalf of the people of Ireland, may I express my deepest sympathies to all those bereaved and affected by this brutal tragedy.
"Our thoughts in Ireland, and of the Irish in the United States, are with the people of Nevada and the United States at this tragic time."
Foreign Affairs Minister Simon Coveney condemned the shootings and expressed condolences on behalf of Ireland.
"This was a savage and brutal attack targeting innocent festival-goers," he said. "I wish to express my heartfelt condolences and those of the Irish people to the families of the victims, those who were injured and to the people and the authorities of the United States on this tragic day."
Irish diplomats in the consulate general in San Francisco and the honorary consulate in Las Vegas are monitoring the situation.
The Department of Foreign Affairs issued a contact number for families in Ireland who have concerns about relatives to ring the consular assistance unit on +353 1 408 2527.