Dublin rental car and drop off in Belfast

There will be a charge for renting a car in Dublin and dropping it off and Belfast. 2 different countries.

Best you contact the rental car company to see what extra insurance might be needed.

From Google:

Key Considerations:
Cross-Border Fees:
Most rental companies will charge a fee for driving into Northern Ireland. This fee can vary, so it's essential to check with the specific rental company.
One-Way Rentals:
If you plan to drop off the car in a different location (e.g., pick up in Dublin and drop off in Belfast), you'll likely face an international one-way fee, in addition to any cross-border fees.
Insurance:
When driving into Northern Ireland, you may need to arrange for additional insurance coverage, as your rental company's insurance might not be valid by default.

A quick look shows a one way drop fee of $150-$250 will apply. Varies depending on car class and rental company.

Up to you to decide if the convenience is worth the cost.

Why would you not just fly from DUB to EDI?
Better yet, arrive DUB, spend time there, take the train to Belfast, spend time there and THEN pick up the car at BHD and begin your tour, ending again at BHD for flight to EDI. The only potential flaw in this plan is that I have heard that the cross-border fee is higher when you rent the car in Northern Ireland. I don't know if this is true, of course.

Wondering if easier to just drive the extra distance from Portballintrae (staying here for Giant's Causeway and northern coast time) back to Dublin airport vs. drop car off in Belfast and fly to Edinburgh from there. Belfast is closer and less time. Looking at my options. It being a one way rental and two countries makes it more complicated. Vacationing in March so I have time to decide.
Curious how other plan and handle this?

I think I am starting to see what the issue is. You use a different type of clock.

Plan now is to explore the island counter clockwise starting in Dublin and fly out of Belfast

Are you not actually going clockwise?

Is there a meaningful difference in airfare to Edinburgh from Dublin vs Belfast? If not, it may be worth driving the 100 miles from Belfast to Dublin to drop the car at the origin and avoid a $150+ one way drop fee. The cost of fuel, assuming 20mpg, would be ~$35.

You just have to price out the options for your specific dates and number of plane tickets. And account for a couple 'extra' hours of driving to see what makes sense ( for you) cost wise and time wise.

Happy travels

Or, if flying from Northern Ireland, why not fly Easyjet to Edinburgh from Derry airport rather than using either Belfast airport?
Or back across the border to Donegal for Loganair to Glasgow.

Hey Joe.. Oops I sit here corrected! Yes, I guess it is clockwise after all isn't it. :)