Understanding you are asking for a place that will be located such that it will be easy to get to Shannon for the next morning flight (without being exhausted from getting up super early to drive a far distance to Shannon.
My suggestion would be to turn in your rental car the evening before, then stay in Bunratty. The Bunratty Castle Hotel is where we stayed for our last night in Ireland a few years ago, and while it is not a super fancy hotel..."somewhat" like a Hampton, we found it quiet, clean, and a helpful staff.
Arrange for your taxi thru the hotel the evening before, and it will be there and ready for you to go.
While in Bunratty the afternoon prior, you could enjoy the Bunratty Folk Park, which would be an easy walk for your mom. As my Irish cousin described it to me, it is as close to what Ireland would have been like when your grandmother (cousin's aunt) was a little girl. It is a charming little place, with thatched houses, shops, church, etc. and the most beautiful, friendly Irish Wolfhound, along with several pleasant farm animals.
I noticed Rock of Cashel was recommended by a poster (and it is a fabulous place that we enjoyed based on a RS poster, I think it was Nancy), my only concern would be to make sure your mom is physically fit to climb up the semi-steep hill to enjoy the venue. It is not a really long climb, but it is a climb....and then, of course, reverse going back to the car park.
