Aron Islands - How much time?

Coming from Dingle, it will take you pretty much the whole day to get to Galway (I assume you'll want to sto a couple times to get out of the car and do a little sightseeing on the way). So that day is pretty well spent.

For me, Galway is a pleasant enough town, but it doesn't really pack any major sights or activities -- it's just a nice place to relax, walk around a bit, eat, sleep, maybe shop a little. I'd trade any day in Galway for a day on Inishmor, which is a special place.

So, I'd get up first thing on the morning after you arrive in Galway, and catch the little plane on the first flight of the day. It'll still get you there long before any ferry day-trippers arrive, so you'll have a head-start on the day-trippers. Spend the day, then I'd spend the night on Inishmor, rather than just doing a day-trip. In fact, I'd then spend the whole next day there, too, and spend a second night on Inishmor. Then, after spending two nights, take the plane back to Galway.

What are your plans after Galway? (If you're headed to Dublin from Galway, you could drop your car in Galway when you arrive, rather than pay for it and to park it while you're on Inishmor, and take the bus back to Dublin from Galway).

Galway isn't one of our favorites either, but after the long days drive up from Dingle , especially if you take the scenic coastal road by way of the Tarbert Ferry, it's probably about as far as you'll want to drive that day.
Given the choice between a Sunday or Monday morning flight I'd opt for the earlier departure on Monday. That would comfortably give you most of the day to explore the island. Though it is a beautiful place and staying over one night would allow you to enjoy the ambience once the day trippers have departed, I think just spending the day would pretty much satisfy your curiosity about the place.
Depending on your available time and interest, I'd then suggest continuing on from the Connemara airport to the very pretty market town of Clifden, which would be a great overnight (or 2) stop, and which would place you in a good position to explore the Connemara Loop Drive the next day, maybe to include the Sky Road just west of the town which is one of Ireland's great scenic drives on a sunny day. The Connemara Loop from Clifden would include stops at the National Park about 20 minutes up the road where you can make the easy hike up to the top of a feature called Diamond Hill, from which you'll enjoy some of the most spectacular views in the region. Continuing along to Killary will allow you to see Ireland's only true Fjord before turning back towards Kylemore Abbey and on to the village of Cong, which still looks like the movie set it was back in the 50's when the old John Wayne/Maureen O'hara film "The Quiet Man" was filmed there - it's a very pretty place to just wander around. Nearby is Ashford Castle - another very pretty place where you could get lunch or high tea without breaking the bank. Many here rave about their "Hawk Walk" as well. It'll be a long day but a pleasant one at a moderate pace.
As was mentioned, if your next stop is Dublin then returning your car in Galway and catching the train back might save you a few Euros.

On our last trip (2019) we took the 1pm Rossaveal ferry arriving in Kilronan for lunch, then a hike around the harbor towards the airport, then dinner and music at Joe Watty's. Next morning we rented bicycles and pedaled out to Dun Aonghasa, had lunch at Teach Nan Phaidi, and returned to Kilronan via the Black Fort, leaving enough time to buy a few sweaters and get on the 4pm ferry back to Rossaveal. We found that the 24 hour visit was plenty of time to see what there was AND it gave us lots of time without the day tripper crowds. We originally planned on arriving from Doolin, but the B&B owner refused to accept reservations for arrivals via that route due to frequent weather cancellations. Only the puffins of Skellig Michael were more enjoyable than this part of our trip.

I agree with others that Galway is a good stopping point/overnight but the real action is out at the Aran Islands. We stayed two nights on the islands and loved our time there. If you rent a bike and you have never ridden an electric bike, give it a try. It was so much fun and I felt like I was 30 years younger when I whipped up hills.

David - I really like your idea of dropping off the car in Galway once we arrive and spending 2 nights in Innishmore. We are paying a LOT of money for car rental (about $185/day) and would love to cut out a few days.
Would you mind taking a look at our schedule and advising us?

Original Schedule:
Aug 20 -- drive from Dingle to Galway - seeing sights along the way - check into B&B
Aug 21 - Explore Galway and surrounding area
Aug 22 - fly to Aron Islands for the day
Aug 23 - Drive to Trim and spend night there.
Aug 24 - return car to Dublin Airport and spend night at an airport hotel
Aug 25 - fly home.

New Possible Schedule:
Aug 20 -- drive from Dingle to Galway - check into B&B
Aug 21 -- Explore Galway - We could either drop off car the night before and just take cabs or return it tonight (Hertz has a center right in Galway) Do you think there are some things to explore that we should have a car this day or just use cabs?
Aug 22 - Fly or ferry to Aron Island (Inishmore). If we stay 2 days, there's not the sense of urgency to get there early. And we will have all our luggage with us. Would ferry be best? Check into B&B. Can you recommend some? I'm wondering if it will be a problem to find one at this late date!?
Aug 23 - take the tour around the island
Aug 24 - Ferry back to Galway - take bus to Dublin Airport Hotel
Aug 25 - Fly home.

What do you think? Does it make sense or am I overlooking something?

I stayed in Glamping cabins at Inishmore. Short walk from ferry dock along the shore. Nice cabins with private bathrooms and small kitchens.
https://www.aranislands.ie/inis-mor-inishmore-island/inis-mor-island-accommodation/aran-camping-glamping
There is a video part way down the web page.

@bherrington47 - Inishmore is a small place (that is, not a ton of infrastructure); choices for accommodations, food, etc. are limited, and you would be booking pretty late (but most people just come as day-trippers, so that's on your side). That said, I just did a quick peek at booking.com and airbnb.com, I do see a few options (literally, just a few, so you won't be able to be too picky). I stayed at a different B&B, so am not familiar with those that are available; you should take a look and see if they align with your needs/tastes/budget.

I see no need for a car once you reach Galway (unless you have plans to see something outside town). Galway itself is absolutely walkable. The little plane flies from their own airstrip outside of town, you buy a ticket to/from the airstrip on their own bus when you book your air ticket (ask); IIRC it was around 10€ for the bus (it's a vintage "heritage" bus). So once you reach Galway, no need for a car (no need to pay for one or worry about parking, either). When you're done with Inishmore, you can catch a bus or train back to Dublin (no reason to drive).

Your updated schedule looks fine to me. You could spend a full day in Galway if you want to, or you could just use Galway for sleeping on the way in and out to Inishmore. You could spend a day in Galway either before or after your side-trip out to Inishmore. Or you could spend that day somewhere else along your route if you short-changed anyplace.

Honestly, I'm not sure you really need a guided tour of Inishmore; we spent 3 days/2 nights (part of the first and 3rd day, all of the 2nd day) there and managed well enough on our own with a guidebook (we are not big on guided tours).

I don't believe there are cars to rent on the island. There are E-bikes. That's what we rented, it helped a lot (there are hills and it is often windy). There are "taxis" of various kinds (horse-drawn carts and cars). The airstrip folks drop you at your accommodations in a van after you arrive. Expect to do more than a little walking (and biking if you're up for it) as you explore the island. We loved it - it was a highlight of our trip.

Tip: When you book the flight, and when you check in for the flight at their airstrip, ask if you can get the right front seat (it's the copilot seat). Some lucky person gets to sit there, might as well ask for it...best views you'll ever have from an airplane if you're lucky enough to get it (flight is less than 10 minutes, so enjoy it while you can). Hope that helps.

It sounds really interesting but my biggest concern is getting around. It sounds like most people ride bikes. I'm 75 and haven't ridden a bike since I was a kid. I also have a bit of a balance issue so I can't imagine riding a bike. I guess anywhere we want to go would be by taxi or walk. Just trying to imagine if it's worth it.

There are lots of posts/threads on Inishmore, it's worth looking through some of them for opinions/descriptions of the challenges & rewards of getting around. Here's one worth a look: West Ireland sights for seniors

We stayed at Clai Ban B&B which is about 50yds from Joe Watty's Pub.
There are shuttle buses everywhere and horse drawn carts, both which will take you where ever you want to go. We were only 70 at the time, and conventional bikes were fine. E-bikes would be even easier.