Letterkenny to Giants Causeway and back

The car rental companies allow you to drive from the Republic of Ireland into Northern Ireland with a supplemental fee of about €30 and up. Check www.AutoEurope for rentals.
The weather in March will have highs of about F49 degrees, lows about F36 degrees. It’s cloudy about two-thirds of March with less than 3” of rain falling during 14 out of 31 days. Basically, on average, It’s cloudy, damp & cold in March.

You would bypass Derry anyway, so should really take that opportunity to visit the City.

Then rather than take the direct A2/A37 route after Derry take the Causeway Coastal Route. You could even extend along the coast towards Belfast then back direct.

More details of the AONB you are passing through on the Coastal route.

Although it is an Open Border you will still need the new UK ETA- https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

I see you are going on to Edinburgh and Northumberland anyway afterwards, so now is your chance to get that ETA anyway.

Thanks for this info (and for the help with Edinburgh--I booked our tour!) I will take a look at that route and your suggestions. We had planned to do a day trip via bus (possibly with my son) to Derry as it's only 35-ish mins away. I won't need an ETA (Irish citizenship) but I was really glad to learn of it as my daughter (who is going with me) will need it!

I drove this exact route in Oct. 1997. I had no issues renting a car in the Republic, driving it into N. Ireland and driving the car back the same day. You'll pass by Derry, Northern Ireland, a relatively big city that's definitely worth seeing if you have the time. The drive from Letterkenny to Giants Causeway would have taken 2 hours direct, but I stopped in Derry. I think the amount of daylight in October and March are roughly the same.

The Giants Causeway has a famous footbridge. When I visited, it was closed because of heavy seas. Other than that, there were no issues.

The Antrim Coast is spectacular. While you are near Giants Causeway, which is so fascinating, be sure and visit the Carrick A Rede Rope Bridge, and the Dunluce Castle and Bushmills Distillery. And while in Derry take the Darren McCrossan City Walking Tour. Enjoy!

Excellent! Thank you everyone for the advice, suggestions!

You're right to use the bus to go to Derry, as parking can be a challenge and everything is very close by. Don't miss the Museum of Free Derry and the Bogside murals.