Apple needs to keep it's international after tax profits out of the US or it will be forced to pay another 40%+ in taxes on them. So it puts them in Ireland, because Ireland gave it a great tax rate, less than 1%.
What should the tax rate be on the interest from billions in savings? Apple isn't' forcing Ireland to build roads or spend any government moneys to allow it to make Irish bank deposits. it's a completely legal arrangement for both Ireland and Apple.
It's crap like this that forced the UK out of the EU. Nothing the EU does is going to force Apple to move it's savings to Germany or France so they can impose high taxes on their interest. That cash will go to Asia first.