You didn't do anything wrong - perhaps reply saying that the car was assigned by a National agent without any request from you. If you were in the US, I would suggest a call to Customer Service but you are in Ireland. You definitely should not be charged differently because they assigned you a car that probably is a slight upgrade from what you booked (could be that they didn't have any on hand - basically an opup in airline speak).
Most of us here in the forum are Emerald Club (or Executive / Elite) so we aren't really that familiar with this type of situation.