Per Diem is a fixed amount for Contractors, fly/no-fly. . . . very different scenario for permanent guys.
The new legislation concerns social charges, not taxes.
AFAIK it applies from 1st June , but the door has been left wide open & anybody employed before this date can continue on the "old" system (social charges paid in Ireland for Ryanair) for 10 years before implementation is required.
New employees (or anyone changing "home base") are supposed to adhere to new legislation.
We have no definitive news on this thus far.