The US embassy in Dublin is inviting people to share messages of support on its Facebook page.
The embassy is also flying the US flag at half-mast.
At least 59 people were killed and 527 others injured after a man opened fire on a concert from the window of his hotel room.
Officers recovered 19 firearms, explosives and ammunition from Stephen Paddock's home in Mesquite, Nevada, hours after discovering 23 guns in his room at the Mandalay Bay Hotel and Casino.
Police also found several pounds of ammonium nitrate, a chemical used to make explosives, in the 64-year-old's car.
Some of the firearms recovered from his hotel room had scopes, while two were reportedly modified to make them fully automatic.
Las Vegas Police Department said officers were searching another home owned by Paddock in Reno, Nevada, adding that authorities were still unclear about the gunman's motive.
"We're hunting down and tracing every single clue that we can get on his background," said Sheriff Joe Lombardo.
"So far, investigators had found no manifesto or anything else to explain Paddock's actions."
He added: "I can't get into the mind of a psychopath at this point."
The Chargé d'affaires at the US embassy, Reece Smyth, has thanked Irish people for their support so far.
"I want to thank all our Irish friends who have expressed their condolences to the victims and families of the terrible attack in #LasVegas", he wrote on Twitter.
— Reece Smyth (@USAmbIreland) October 3, 2017
The embassy has also posted a message on its Facebook page, inviting people to leave messages there.
"The US embassy joins all Americans, Irish, and people around the world in expressing our condolences to the victims and families of the horrific attack in Las Vegas.
"If you’d like to leave a message of condolence, please do so below."