The Danish and Irish tax authorities have reached an agreement, which ends a 1965 agreement on double taxation. According to the old agreement, which Ryanair staff based in Denmark is currently working under, taxes was paid in the country of the operators home base. The new deal, however, has changed that so that employees pay taxes in the country they are based.
The reasoning behind this deal is to end social dumping, as the tax rate in Ireland for low-wage earners is around 20% vs around 40% in Denmark. By offering staff a contract at the lower Irish tax rate, Ryanair has been able to pay substantially less than the market average. And, of course, at the same time having their employees leeching off a society they don't contribute to.
Link (in Danish):
Skatteminister slukker for Ryanairs danske skattefidus | Nyheder | DR