A big fine, worth around €3250 to each Irish citizen.
The US are already unhappy because Apple plays as little tax as possible in the USA and is now daring to pay some in Europe too. Not that Apple will mind, might be quite a relief given the huge cash they have built up and limited investment opportunities open to it.
What of Ireland though...the EU is clearly stepping in, rightly, to stop a sweetheart deal in an economic zone of freedom of capital movement. Other companies may be dissuaded from signing such deals and moving there in the future.
Which all makes sense in the EU zone. It does present an opportunity for the UK; attract Apple with a low rate sweetheart deal and get them to base in the UK post-Brexit. Exactly the kind of mercantilist policy Germany has been using for years via the EU.
Interesting choices for Ireland coming up, its corporate back taxes, it should go to Ireland, but no doubt the EU will try and claim it as a fine for the EU coffers.
All this on a day when France followed Germany in pulling out of TTIP trade negotiations with the USA too. Brexit shares still moving up, EU ones down.