Here's an answer from a local:
I'm an American living in Waterville. Keep in mind your Skellig Michael boat trip might be cancelled (many days they were cancelled due to weather, lately) so you can explore that day near PortMagee if the boats don't go out.
Kenmare to Waterville is the prettiest part of the Ring of Kerry in my opinion. You won't want to drive quickly. You will, it sounds, be driving off peak as the buses will be gone after 4:30-5:00 p.m. I suggest you stop at Parknasilla Resort if you have time for a walk - it has 6 walking trails, easy for non-hikers, and is beautiful. - it's right on a little bay. A gem.
If you see a sign "Tra" that is Irish for BEACH. If there's a sign, there is usually parking (though it might be for only 2 cars!). You WILL drive by quite a few lovely beaches on that part of the road. O'Carrolls Cove (signs say "Only Beach Bar in Ireland" or something!) is fabulous. Tropical fish like Florida on summer days! Worth a paddle as it's protected and calm most days. Water clear as gin! As mentioned above, Derrynane is spectacular. Turn off at Caherdaniel to get to Derrynane. It's the 2nd biggest national park (I think) and worth a wander. The Daniel O'Connell House will likely be closed at that hour. If you like walking, check out my blog post on walking the Mass Path at Derrynane recently.
There are a few RING FORTS on this drive. Staigue Fort (big, lots of tourists); Loher/Lohar Fort next to Waterville, and another near Cahersiveen (so you can save for the next day).
After you pass Waterville, turn left on the road that says "Skellig Loop" and "Skellig Ring." It's the long way around to Port Magee, but going around that peninsula is beautiful and often missed. If you're uncomfortable with driving a stick shift with your left arm, know that the road near Finan's Bay is steep, wiggly, and narrow for 2 way traffic.
The Kerry Cliffs are spectacular! Do not rush these! I send every tourist I meet, there. I've been over 15 times myself. You're in a good spot!
Enjoy!
Susan