Paul636, thank you so much for your information and links. I didn't know we had to reserve a specific time slot for Newgrange. That gives me pause about doing that. Generally, on our last sightseeing day, especially since we are driving from one destination to another, we like to wing it sometimes. Decide while driving where we want to stop.
caparrish, thanks for your Trim Castle recommendation. Looks like a winner! We don't have to pre-book a specific time slot for Trim Castle, do we?
Since my husband is the driver, I will go over everything with him. He'll want to map out drive times, etc. to see what makes the most sense.
I like everyone's suggestions so it will be hard to know what to eliminate. I know we want to keep it easy and stress free. Because this will be our last sightseeing day in a 3-week trip, I know we will be losing some energy. We have to return our rental car that day, too, by 7PM. But knowing us, we will probably want to return it by 5PM.