As much as agreed to in treaties when joining the EU.
Here is a quick explainer:
"Ireland is a member of the European Union, which operates something known as the single market, allowing companies based anywhere in the EU to seamlessly sell goods and services to other countries in the EU. This single market means that many regulatory issues and other important government functions are handled at the EU-level, through the Union’s permanent bureaucracy in Brussels... The European Union does not have a unified taxing authority. But it does have a unified anti-trust agency, run by the competition commissioner. This is a very important agency because, by tradition, busting up national monopolies and ensuring continent-wide competition was one of the primary missions of the European Union. As part of that, the EU attempts to restrict member states from subsidizing particular companies. The specific fear was that the government of, say, France would direct subsidies to France-based automobile companies like Peugeot, thus unfairly disadvantaging Fiat unless Italy also stepped up with subsidies. Consequently, this kind of state assistance to firms is considered anti-competitive under EU law. But in recent years, Vestager and her competition commission have interpreted favorable corporate income tax deals as a form of illegal subsidy. She says that due to its arrangement with the Irish government, “Apple only paid an effective corporate tax rate that declined from 1% in 2003 to 0.005% in 2014 on the profits of Apple Sales International.” This, she says, is “illegal under EU state aid rules, because it gives Apple a significant advantage over other businesses that are subject to the same national taxation rules.”
Why Europe is ordering Apple to pay Ireland $14.5 billion in taxes Ireland doesn’t want - Vox