I cannot comment on Dublin as we flew in and out of Shannon. We were very pleased with our choice. The Shannon airport is small but very efficient. Public transport was right outside the door. (we didn't have a rental car) The best part of all was going through US customs at the Shannon airport. They have a separate large room just for that purpose and it appeared they only allowed entrance to one flight at a time to that area. As a result we were the only two people in the entire area going through US customs. Quite astounding experience compared to entry points within the US like SFO, Newark, Dulles, etc
If it were me I would return the rental car a day ahead and stay at the Bunratty Castle Hotel. It is only a few miles from the Shannon airport. We arrived early to the hotel, checked in and walked across the street to the Bunratty Castle park. https://www.shannonheritage.com/BunrattyCastleAndFolkPark/
It was a lovely way to end our trip. We spent a few hours strolling the park , looking at the exhibits and having a pint at the pub in the park. Lovely small shops inside the park with homemade items from sweaters to pottery and jewelry. Outside the park is a woolen mills outlet. We had a "senior" package that afforded us dinner at the hotel. The next morning the hotel called a cab to take us to the airport. It was inexpensive- under 20 Euro. This plan created a lovely reflective ending to our trip and made the beginning of the trip home very easy and stress free.