A spokesperson for the company, which operates five bars here since entering in 2014 and has plans to double that number in the next 18 months, was unable to confirm whether Irish customers were caught up in the attack that occurred in June but was only uncovered on Tuesday.

Some 655,000 customers have been affected, but only 100 or so who bought Wetherspoon vouchers online before August 2014 were seriously affected by the breach.

These customers had some card details stolen but not enough to be used fraudulently, according to the firm.

“We apologise wholeheartedly to customers and staff who have been affected.

A database containing the details of 656,723 JD Wetherspoon customers has been hacked https://t.co/vpZPkhMSjm pic.twitter.com/s0Eiqkx58r — SkyNews (@SkyNews) December 4, 2015

"We are determined to respond to this by increasing our efforts and investment in security and will be doing everything possible to prevent a recurrence,” said Wetherspoon chief executive John Hutson.

Some personal details of staff registered before November 2011 were also stolen, but no salary, bank, tax, or national insurance information was accessed.

Wetherspoon has alerted customers to the situation by email and informed the information commissioner’s office in the UK.

Wetherspoon gathers customers data in a number of ways including through people signing up to its company newsletter.

The information was held on the company’s old website which has since been replaced.