he'll still have to pay taxes to america (wierd tax policy from america btw)
He does not have to pay taxes, this is very unlikely, if you flee to another country you are not forced to pay taxes of the country you were from before. The only thing he may face is a trial when he comes back to the US, but as long as he stays in Ireland (or elsewhere) and have a permanent residence or double nationality granted then no worries.
And again if this was possible then the rich people fleeing elsewhere just to avoid taxes (like in France) could not avoid to pay these taxes, this would be pretty nice for our economy... but guess what, this is not that easy.