[jurassic]

I don't think it's fair to assume harmonized tax rates are mutually beneficial. Taking Ireland as an example, how many of the companies that shifted profits there for favorable tax treatment would continue to do that if they had to pay the same as they would in the US? My guess is relatively few would remain. So Ireland would likely lose revenue without that discount as an incentive drawing in these companies.

[ineptech]

Without a harmonized tax, Ireland could also lose all of its corporate tax receipts to some other country that's willing to stoop even lower.

More generally, this is a variant of a mutual coordination problem in game theory, in which it is everyone's interest for everyone to cooperate, but in each individual's interest to be the only party that doesn't cooperate. Those are notoriously tricky to induce/enforce agreement on, but it has been done.