I will be in Cong for three nights in September and would like to visit the grounds of Ashford Castle Hotel. Can you do that even if you're not a guest staying there?
Indeed you can. If you go via the front gate, they may ask for an admission fee. If you enter through the back on foot, you can avoid the fee (you'll take the the road from town that goes along the ruined abbey and church in town). You are welcome to wander the gardens and the paths and see the views of the lake.
You won't get very far into the hotel unless you are a "resident" or going for dinner (jackets required), but you could always go to the desk and ask about their horseback riding or falconry school to buy a few extra seconds in the lobby.
The back way in goes through Cong Woods. It is just lovely walking through the trees and along the water. Because of Cong's associations with the movie "The Quiet Man" I suggest you rent the movie. It was filmed in and around Cong and Connemara. It will be fun to try to pick out scenes from the movie when you are there. In the grounds of Ashford Castle if you go through a tunnel in the gardens and follow the signs for the falconry you will pass by "Dannaher's farm" from the movie.