They broke the law, Apple and Ireland. Just because Ireland is a country it doesn't mean it can't be subject to laws.
> "Ireland granted Apple unlawful aid which Ireland is required to recover". The European Commission found that corporate tax rates as low as 0.005% paid by the tech giant represented an unlawful subsidy. Specifically because other companies were not permitted to obtain the same tax arrangements. As a consequence Apple must pay €13 billion, excluding interest, to the Irish Treasury.
> "Ireland granted Apple unlawful aid which Ireland is required to recover". The European Commission found that corporate tax rates as low as 0.005% paid by the tech giant represented an unlawful subsidy. Specifically because other companies were not permitted to obtain the same tax arrangements. As a consequence Apple must pay €13 billion, excluding interest, to the Irish Treasury.