Ireland Itinerary Ideas

We did the 7 day RS tour this past summer. After the tour we spent time in Dublin and took a one day tour of NI with Wild Rover tours. They took us to Belfast where we did a black cab tour the tour had a choice of that or the Titanic museum. The cab took us there so we saw the outside and the gift shop.

Also a BIG Derry Girl fan. Wish we could have gotten there but hard with only a day. The black cab tour was excellent and gave us great insight.
We also went to Giants Causeway and a ruin.

If you don’t want to drive the tour was a great option.

Make sure your rental car you can take into NI. And I know there a new regulations to get into NI in the new year too.

Loved Dingle. Sounds like you will have a great trip

The itinerary seems fine. Note that you might want to leave the car rental in Letterkenny as crossing into Northern Ireland will incur a cross-border charge of about €30 and a one-way drop off charge possibly as high as €200 since Belfast is in the UK. There may also be differences in the insurance coverages required in the two countries. To keep matters simple, I would leave the car in Letterkenny and bus 40 minutes to Derry then bus/train 2 hours to Belfast. There are tours of the coastal points of interest such as Giant’s Causeway out of Letterkenny and Derry.
www.AutoEurope.com

Our rental fee only added 30 Euro for crossing border. Dropping off in different location was more the issue in terms of costs. Will be complicated to hit Antrim Coast/GC without car.

We really liked Derry and enjoyed the walls. We were shut out of the Derry Girls/tower museum exhibit because the museum was closed ) but you need to book timed ticket in advance. They are reasonable.

Derry, GC, Dunluce Castle/other Antrim coast site, would be a very full day on way to Belfast. It's not an insane amount of driving but you won't have a ton of time at each.

We skipped Titanic (kids decided, I regret) but did cab tour (a MUST do), city hall was worth a peek inside if you're staying close. I think we would've benefited from more time in Belfast.

A possible alternative to your proposed itinerary after the 23rd would be to fly from the Kerry airport back to Dublin, collect your car there, and then do your circuit of the sights in NI with more time at your disposal to relax, enjoy and explore. RyanAir has two cheap flights per day that would save you a lot of driving, not to mention the saving of about USD $250 for a one-way drop charge on your rental car.
Belfast and Derry are both terrific, and the scenic drive between them along the Glens of Antrim is one of Ireland's finest on a sunny day. The Ulster-American Folk Park near Omagh is excellent and could be an intermediate stop on your way to or from Derry. Could also include a stop along the way to see Newgrange and visit the sights in the Boyne Valley for a crash course in Irish history.
Since it appears that you may be considering another trip to Ireland in the future I'd suggest slowing down a bit, prioritizing what you really want to see this trip, and saving Connemara, Donegal, and/or Kerry and the Beara peninsula for a future visit.

You will need the UK ETA to visit Northern Ireland even though it is an open border with no border checks- https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

Very briefly, from Western Dingle, I personally could (and would at the drop of a hat) do the whole journey as described by public transport with a good degree of ease, at least once back in Dingle Town. We don't know where in Western Dingle the start is, but there is a surprising network of deeply rural bus services into Dingle.

Without me spending time on the exercise look the Irish parts up on Transport for Ireland, and the Northern Ireland parts up on Translink.

There is a difference between me (solo) and a family of four doing it, and also between a person like me familiar with using rural Irish public transport (and indeed doing so in many places round the world including parts of the US), and people less familiar (and probably not familiar with doing so at home either).

Because of the high cost of dropping the car in Belfast, I would either skip it or return the car (after using it to visit the Giant's Causeway) in Dublin and "double back" to Belfast by bus or train. You could even leave the fam and bags there to work and just send one person on this mission.
I went to St. George's Market in July and didn't last 10 minutes in there. They seemed to be mainly selling tourist crap I could get anywhere (should I want it).
Derry is worth your time. The Museum of Free Derry and Bogside Murals are a great half-day outing, as is walking the walls, with or without a guide. It's a small city and very easy to navigate on foot. There is lively bar scene there (though I never found a pub with a menu that didn't read like the one at Applebee's).

All - Thanks so much for the replies! Lots of great insight and suggestions.