Ballycarbery Castle is a 15h century ruin that is now beautifully covered in ivy, and in a gorgeous lowland setting with little development around. When I was there, we were able to go right up to the castle and even climb inside a bit where we had a picnic. Apparently this was once a magnificent castle, and its proximity to Caher Gael stone fort makes the visit doubly worthwhile. Both sites can be easily incorporated into a Ring of Kerry trip.
There was no admission fee when we were there. As with all these ruins, it's not necessarily safe to get inside, so use your best judgment.
This 15th century tower house is situated in the near of Cahersiveen, off the famous Ring of Kerry. The ruins, partly covered with ivy and situated close to the sea, look great!