Names, email addresses and passport numbers are among the details affected by a data breach at the Marriott hotel group, which owns Dublin's Westin Hotel.
The data breach occurred after the guest reservation database of the chain's Starwood division in the US was compromised. A staggering 500 million guests' data may have been exposed during the breach, which began in 2014 and was spotted on September 8 this year. Guests who made bookings before September 10 this year may be affected.
The database stored information including passport numbers, dates of births, names, addresses and phone numbers for 327 million guests. Marriott said some guests' credit card information may also be affected.
Work is continuing, but the firm said the breached database contains the information of up to half a billion guests who booked before September 10.
Payment card numbers and expiration dates were also stored for some.
The Data Protection Commissioner's Office hasn't received any official notification of Irish customers affected yet.
However, it'll be making contact with Marriott International to see if any have been affected.
Some of the hotels which were hit in the UK include:
The breach was spotted in the Starwood guest reservation database in the US on September 8 and the company "discovered that an unauthorised party had copied and encrypted information, and took steps towards removing it", a statement said.
Security experts determined there "had been unauthorised access to the Starwood network since 2014", it added.
Researchers decrypted the information and determined its contents were from the Starwood reservation databases on November 19, Marriott said.
Marriott president and chief executive Arne Sorenson said: "We deeply regret this incident happened.
The Maryland(US)-based firm, which has hotels across the globe, said law enforcement agencies are investigating.
Payment card numbers are encrypted using a method that requires two components to break it, a statement said.
"Marriott has not been able to rule out the possibility that both were taken," it added.
The US National Crime Agency said it is making inquiries, and the New York Attorney General has also opened an investigation.
The UK's Information Commissioner's Office (ICO) has begun making inquiries over the breach and has the power to impose large fines.
"We have received a data breach report from Marriott Hotels involving its Starwood hotels and will be making inquiries," a spokeswoman said. "If anyone has concerns about how their data has been handled they can report these concerns to us."