They don't. What Ireland has signed on, is that, just as in any EU country, the corporate tax must be equal for all. National or local governments cannot negotiate deals with specific companies. This is a promise they have broken, and the cause of all the noise about Irish tax.

Seeing so many people discussing this under wrong assumptions, even on HN, makes me sad and makes me think there's a very low ceiling on tax complexity, above which people lose track completely. This means we'll never be able to fix tax. Only tax lawyers will ever even begin to grasp it. It's also a pretty good argument against direct democracy as well as an argument for abolishing the corporate tax completely.