The European Commission said its preliminary findings suggest Ireland provided unlawful state aid to tech giant Apple for more than 20 years.
In a letter published today, European Commissioner Joaquin Almunia told Ireland it must provide details of the tax arrangements.
Almunia said any unlawful aid could be recoverable from Apple.
An embarrassed Apple has released a new update that the company says will repair the problems caused by software it released earlier, repeating an apology to owners of its newest iPhones affected by the bug.
The tech giant had scrambled to fix the software glitch that left some of its new iPhones unable to make calls, but some analysts say Apple's response to the high-profile gaffe may be more important than the fault itself.
Hours later Apple released a new update, dubbed iOS 8.0.2, which it said would fix the problems caused by the iOS 8.0.1 update that it released on Wednesday.
Apple has said that only nine customers have been in touch to complain about a bent iPhone 6.
A spokesman for the IT giant told CNBC that it had selected "high quality materials" and that the new models have been through rigorous tests.
CNBC editor Jon Fortt tweeted:
Apple: w normal use a bend "extremely rare & through ... 6 days of sale, a total of 9 customers" said they had a bent phone @cnbctech
Apple used rigorous tests throughout iPhone 6 dev cycle: 3-point bending, pressure point cycling, sit, torsion & user studies @cnbctech
Apple has apologised for the rotten experience reported by the buyers of its latest iPhone.
Some people who raced to get the iPhone 6 Plus last week have since complained that the smartphone bends out of shape in their pockets.
Widespread user discontent followed about problems with Apple's upgrade to its new iOS 8 operating system, prompting the company to pull a release that had been issued to fix existing errors.
ITV News Consumer Editor Chris Choi reports on a difficult few days for the tech giant.
Computer giant Apple has apologised after an update of i.O.S.8.1 left some iPhone6 users without service and other problems.
They said: "We apologise for the great inconvenience experienced by users, and are working around the clock to prepare iOS 8.0.2 with a fix for the issue, and will release it as soon as it is ready in the next few days.”
They added that they had a workaround for iPhone 6 and iPhone 6 Plus users which enabled them to restore the phone service and Touch ID functionality after updating to iOS 8.0.1.
They said: "Affected users can reinstall iOS 8 through iTunes. For more information visit http://support.apple.com/kb/HT6487."
Apple pulled iOS 8.01. after users reported problems after downloading the update onto their iPhones.Read the full story ›
By: Alok Jha - ITV News Science Correspondent
The bending rumours are unlikely to put any die-hard Apple fans (or many others) off from buying their new iPhones but if you’re worried, it seems sensible to exercise a little common sense when using it.
As Rik Henderson wrote for Pocket Lint: "Don't put a 5.5-inch superthin phone in your back pocket, no matter whether it is made by Apple, Samsung or any other manufacturer.
“You wouldn't put your glasses in your back pocket would you? And they probably cost less.”
Angry owners have claimed the new iPhone 6 Plus has bent in their pockets just days after buying them in what has been dubbed 'Bend Gate'.Read the full story ›
Apple has sold more than 10 million of the new iPhone 6 models in the first weekend - a record total that could have been higher if more stock had been available.
Tim Cook, chief executive, said that sales of iPhone 6 and iPhone 6 Plus, had exceeded the technology giant's expectations. It took more than four million pre-orders for the new smartphones, which boast larger screens.
"While our team managed the manufacturing ramp better than ever before, we could have sold many more iPhones with greater supply and we are working hard to fill orders as quickly as possible," he added.
Apple sold five million iPhone 5 handsets, four million iPhone 4S models and 1.7 million iPhone 4 units during the first weekend.
Actors Tom Hardy and his fiancee, actress Charlotte Riley, along with Mamma Mia! star Dominic Cooper and Doctor Who's Jenna Coleman were among the thousands who picked up the new iPhone 6 today at Apple's Regent Street store in London.
Reports suggested that some 5,000 people queued outside the Apple store on Fifth Avenue in Manhattan, while outlets in the 10 countries were the device went on sale today were all busy with early adopters. Last week Apple reported a record four million pre-orders for the iPhone 6.
Most technology writers have praised the new smartphone. Stuart Miles, founder of gadget website Pocket-lint, said: "Ultimately the two new iPhones will be very successful because while Apple doesn't deliver new tech it delivers it in such a way that consumers want without getting bogged down with tech specs.
"Apple's attention to detail has a huge knock-on effect in making other manufacturers continually up their game."