I didn't go inside the house; it's cheap, but I didn't think it was worth it. The gardens are spectacular for strolling. People bring their pets to the surrounding area, and there's parking. Depending on the access route, the roads are better or worse; some only fit one car at a time, and there are widened sections for passing vehicles. There's a short access road and a longer one.
We went specifically to see the Fairy Trail: it wasn't easy to figure out how to start it; Google indicates it at another entrance, but to be sure, it's best to follow the signs for Derrynane House, and shortly after entering the estate, there are signs for the Fairy Trail. A beautiful walk.
This place is magical. The surrounding walks through woodland and amazing beaches are spectacular. Abbey Island is a serious bonus. The house is beautiful. And then there’s the history! The cafe serves great coffee and a lovely selection of cakes. A must visit, in my view.