Where to spend last night- between Cashel and Dublin airport? What not to miss en route?

I'd recommend Glendalough, a glacial valley and monastic settlement some 20 miles or so south of Dublin, in County Wicklow. It has dozens of antiquities: https://glendalough.ie/

I agree - Glendalough, and then the scenic drive thru the Wicklow mountains back towards Dublin.
The visitor center at the monastic site closes at 4:30 PM but the site itself is always open. After the day trippers depart you'll have the place all to yourselves.

It would help to know when you're going. In summer, it stays light pretty late, so Glendalough could be an option.
I would also take a look at Athlone, which has a castle and an old pub and the nearby Clonmacnoise.

That sounds lovely. Are there pubs or things to do in the evening? Or would this just be a stop on the way to stay elsewhere (Dublin or Trim or ?)?

We are going in July.

When I was last in Glendalough in 2002, there was a pub at the crossroads -- whose name I cannot remember -- with a very authentic Irish feel and comforting fire. If there's enough light out, you could go hiking in the evening as Glendalough is made for hiking.

Between Glendalough and Dublin there's a famed pub in Glencullen, Co. Dublin, called Johnnie Fox's with Irish dancing: https://johnniefoxs.com/. I haven't been there, but people rave about it.

I've never stayed in Glendalough although I've been twice (and highly recommend). But I have been to Johnnie Fox's. The dinner show is very fun with fantastic music. They also have trad music nights.

Alternatively, look at Avoca a small town in the Wicklow mountains a few miles south of Glendalough.

Although it will be crowded in July, there's Kilkenny. Under 2 hours to DUB.

In August 2023 our family group went to Curragh Racecourse to watch the horse races which was a very fun and memorable afternoon. That evening we had dinner and stayed at Lawlor's Hotel in Naas. Would definitely recommend.

While certainly agreeing that Glendalough is a wonderful place to visit, I would also suggest an overnight visit to Dalkey, south of Dublin. A beautiful village with numerous fine eateries and wonderful pubs. I would especially suggest Finnegan’s for a pint (or 2?). A fine old castle worth a visit. A walk along the coast. Very picturesque. And very easy drive to the Dublin airport, considerably closer than from Glendalough. That would allow for a leasurely breakfast and still plenty of time to get to the airport for your flight home.

We stayed in Trim for our final evening, arriving early the day, and found it to be a convenient location for an easy drive to the airport early the next morning, without worrying about a delay on the M50 due to a traffic accident (which can slow travel to a crawl). We stayed at Caravogue B&B, had a wonderful time exploring Loughcrew (cairns, lots of sheep, and fabulous views), enjoyed the river walk at Sheep Gate, and had a wonderful dinner at The Stockhouse. There are lots of sights in the entire Boyne Valley, but I see that you have part of that covered in Day 4. Everything about Trim and our stay there was well worth it. All the best with your travels.