One-way car rental fees

This is Rick's advice on the subject:
http://www.ricksteves.com/travel-tips/transportation/car-rental-costs
scroll around, he'll probably cover your point.

No, there is really no way to avoid the fee. Your issue IS ("about renting a car in one country and dropping it off in another"). Dublin and Belfast are in two different countries.
There is no way a car hire company could rent out a car registered in a different country. So, they have to drive it back across the border to Dublin, for which they will charge you.

The way to avoid it is to drop the car in Ireland and continue to Belfast by bus or train.

Yes, there are 2 different countries here.

Republic of Ireland. Currency is € (EUR). Capital is Dublin.

Northern Ireland. Part of the UK. Currency is £ (GBP). Capital is Belfast.

Do a little Google search for each country to see maps and learn more about where you are going. The history is both tragic and fascinating.

Realize that it is the car rental company, not AutoEurope that is charging the fee. I rented a car through AE picking up in Madrid and dropping in Lisbon. The customer service rep, on finding that Europcar's drop off fee was $600 said he'd research it and call me back. About 30 minutes later, he booked me with Avis and a $350 fee. I've rented with AE numerous times in the past and feel I've always received great service and good prices. I don't know of any rental company that does not charge a substantial fee when the return is cross border.