Judging by your response you're obviously not familiar with the specifics of this case.
What happened here was that the Irish government made secret deal with Apple where they promised to locate their operations in Ireland in return for charging Apple next to nothing in taxes. The Irish corporate tax rate at the time was already lower than that of most other European countries at about 20% IIRC, but the secret deal they gave Apple was one of a measly
0.005%! Apple then used their Irish subsidiary to do business across Europe paying next to no tax, giving them a significant advantage over their competitors.
Not only was this essentially giving Apple the ability to do business in Europe with effectively no taxes on their profits, this deal was also illegal under the basic articles of the EU. Hence the secrecy and the fact that it had to be leaked before the EU did anything about it. These kinds of special deals are very much illegal under the basic laws of the EU, which Ireland ratified under when they joined the EEC, the predecessor of the EU, way back in the
1970s. Under EU law these kinds of deals very clearly qualify as illegal state aid meant to skew the market and are hence highly illegal.
So in other words Apple went into a no-tax deal with Ireland knowing full well that this was illegal under EU law and tried to hide this by having this deal be secret. However this deal leaked and the EU has mandated that the Irish need to collect their standard business taxes as this very clearly qualifies as illegal state aid under EU law. If Ireland or Apple doesn't like this they're completely free to leave the EU, but as long as they are in the EU they need abide by it's laws.
TL;DR Ireland gave Apple illegal state aid by letting them operate effectively tax-free within the EU and now they they got caught they're both sour over the EU forcing Ireland to follow EU law and collect their standard business taxes on Apple's previously tax-free EU profits.