5 days in Ireland itinerary

My first concern is your flight back to the US. What time is that flight? You preclear US immigration in Ireland so you need to be at the airport 3 hours before departure. It's about a 4 hour drive to Dublin airport from Killarney. A distance I would not be comfortable with travelling on the day of my departure. Is it possible for you to fly to Kerry airport upon arrival in Dublin? Then you could rent a car at Kerry, drive the 20 minutes to Killarney and spend all of your Dublin time at the end of your trip and have a comfortable last day before returning home. If you do spend your first night in Dublin don't bother with the HoHo bus. Just walk to Trinity College, the Ha'Penny Bridge and Christ Church. They're all in the central Dublin tourist area. Plus the walk will help you recover from jet lag faster.

Our favorite place in Dublin is Kilmainhaim Gaol. Be sure to get tickets as soon as they go on sale. It is highly suggested you get to Dublin the night before your flight home and leave three hours at airport as you need to go thru US customs in Dublin. Enjoy.

thanks for the quick reply. We are not flying back to the US, but to another European country (undecided but maybe Germany, Copenhagen, London). Does this make a difference in arrival time at Dublin airport? We are also considering a train/flight to Scotland.

Two hours should be enough time. As a reminder you need the new ETIAS form if going to the UK. (not sure of correct spelling )

It's hard to know what makes sense for you. Here are some of my thoughts as to the pros and cons of renting a car.
Pros: flexibility to stop along the way; flexibility in accommodation (kinda because parking in town can be an issue)
Cons: a car that comfortably fits 5 adults (and their luggage if you're coming from Dublin) is a lot of car to drive on the ROK; even a confident left-hand driver might be so focused on driving as to miss a lot of the scenery of the ROK; renting a car in Ireland is expensive, in my experience
Other options: You don't have to go out to the airport to pick up a rental car. I would look at a private van tour of the ROK where you could set your priorities for stops and meals and everyone gets a window seat.

Just want to clarify the registration requirements for US citizens traveling to the UK or EU. The form and registration for the UK is the ETA, https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta?hidemenu=true. No ETIAS registration is currently required for the EU but may be effective by the time you travel. Although Ireland is part of the EU it is not part of Schengen and it is not expected that the ETIAS will be required for travel to the Republic of Ireland.