Ireland's corporation tax rate is purely an internal matter, even in the single market. In fact, Ireland rejected by referendum the Lisbon Treaty in 2008, and only voted for it the following year after a series of clauses known as the Irish Guarantees were added. One of those guarantees is:

> Nothing in the Treaty of Lisbon makes any change of any kind, for any Member State, to the extent or operation of the competence of the European Union in relation to taxation.

It's legally binding, and ensures that Ireland retained the right to set our its corporation tax rate.

All the other EU nations at the time (some have joined since) unanimously ratified the Irish Guarantees, so if anything, it's rich of them to now complain about an agreement that they willingly and knowingly signed up to.