There is also the concern of how 13B Euros of Apple will-be/have-been seized/escrowed for the duration of the case which may takes years. In case EC loses, will EC/EU pay to Apple for temporary loss of their property ?
The Commission, in simple terms, makes sure that a single market doesn't mean that one country helps companies avoid tax in other countries while still allowing completely free trade.
Ireland agreed to obey EU rules when joining.
Ireland was not following the laws
Yes, like all the other rulings regarding the Article 107, the infractor is the state giving illegal aids.
And the companies are "dragged" because they were given an illegal advantage over others companies and that advantage should be voided
Because if Ireland "loses", then Apple owes 14B Euro. So, Apple is an interested party.