Hold on. Let's think about this logically (and forget the fact that tax related stuff can be confusing!)
It sounds like you're saying he's Irish resident in the year in question, is normally Irish resident (i.e. was here for the last 3+ years), and I'll presume he is in fact Irish (and thus Irish domiciled).
If he's Irish resident, he's liable to pay tax on all of his income in Ireland, just like every other "normal" Irish person who has an Irish job. Why would you think he should be able to pay no tax, just because he was sent to work in a different country for a few months?
There's no free lunch; if he's not liable to pay tax on the income in Ireland he'd be liable on it in the US, but based on what you've said he's rightly being taxed here on it.