late September travel

Are you flying into Dublin and out of Shannon? I assume you will rent a car. Without knowing your interests, it's tough to know what is a must-see for you and your group.
I quite enjoyed seeing 2 of the 3 giant troll sculptures in the Sperrins. https://discovernorthernireland.com/blog/post/the-sperrin-sculpture-trail-a-giant-adventure/ The third one was at the top of a long, steep hike that I didn't finish.
From Derry, we took a bus to Donegal through the Barnesmore Bog Gap, which was very beautiful. I was a little taken aback by these magnificent non-coastal landscapes. It's not what I think of when I think of Ireland.
In Killybegs, Donegal, I can recommend this apartment:
https://www.airbnb.com/rooms/41787724?source_impression_id=p3_1779032070_P364wHVQuCebYaz4
And the chippie across the street. Killybegs is a good base for seeing the Slieve League by both land and sea.
I found Derry expensive for accommodation and very touristic for eating (It seemed like every pub had fish tacos and pasta on its menu). Folks on the Forum often recommend staying in Letterkenny instead. There is parking at the mall in the center of Derry, and you can see most of the town in a single day by walking the walls. The Museum of Free Derry and the nearby Bogside Murals are a must-see in my book. They made an excellent counterpoint to the Black Taxi Tour in Belfast (which it doesn't sound like you plan to visit).

Thanks for the insights - we are flying into Dublin and out of Shannon - Hopefully renting a hybrid - Currently I have mapped out some sights in Dublin, a day trip to Bru na Boinne and maybe down to Gardens of Powerscourt and then maybe to Trim to see the castle. we will head to Belfast and then up and around - I wasn't sure what in Donegal or Mayo to see. thanks again!

We were just in Co. Donegal visiting my son. So much to see there and along Co. Derry and Co Antrim.
In Antrim visit:
The dark hedges (this is just a really cool road so it's more of a quick stop to see and take a few pics)
Carrick-a-reed (trail along coast and rope bridge-the cost is only for the rope bridge)
Giants Causeway (you can take a tour through the visitor's centre for a fee or just park at the hotel and walk around for free. We've done both and found value in both.)
Bushmills (cute town but most known for it's whiskey distillery, which was a nice tour)

In Derry:
Free Derry monument and corner
Free Derry museum (also prebook a walking tour through the museum. Incredibly informative)
Derry Guildhall (part city hall, party historical museum)
Walk along the old city walls
Craft Village (cute shops, lovely cafes for a lunch or a cup of tea/coffee, and an original thatched roof house)

In Co. Donegal (we usually stay in Letterkenny and are able to easily access all of these sites by short day trips - including Derry!):
Malin Head
Farren's Bar Malin Head (this is a great pub with very good food, right by Malin Head. Worth a stop for a bite to eat.)
Fanad Head Lighthouse (the drive up to this lighthouse is extrodinary. Take the tour if you have time.)
Glenveah National Park (lots of hiking and walking trails)
Sliabh Liag cliffs (you can either drive to visitor's center and take the shuttle up to top or park up close to the top then walk. There is a breath taking hiking trail you can do along the cliffs that takes about 1.5 hrs one way.)
Mt. Errigal (I did not know that there was such an impressive mountain in Ireland!)
Killybegs is a great stop for lunch. Cute fishing village, good fish and chips (you will see that most fish comes from here) and there are boat tours that take you around to the cliffs should you choose.
Donegal Town (cute little village with small castle and church)

This is by no means a complete list, just some of the top things we've seen. When we've done various parts of the WAW, we've stopped as we go when we see a sign noting something interesting. We typically stay at Dillon's Hotel in Letterkenny and take day trips to Co. Derry, Co. Antrim and around Co. Donegal. In Galway, we prefer Leonardo's Hotel though there are many good hotels there. In Clifden, I did not like where we stayed so I won't recommend. If you head down to Doolin, we love (LOVE) Sheedy's Doolin Guest House.
Hope you enjoy your trip!

Most of Mayo is off the beaten path. I think jjgurley (https://community.ricksteves.com/users/863527) has been recently to the Ceide Fields and Belmullet. You might think about messaging them for more info on that area (called Erris, I believe). It's on my list of areas to visit next time.

Thanks so much - I will definitely check into these places!!