Slieve League looks spectacular.

It is! It is less busy than Cliffs of Moher and we much preferred it.

The cliffs are indeed spectacular and are relatively uncrowded, as is the rest of County Donegal - Ireland's "wild west" featuring some of the most beautiful scenery in the country. You'll need a car to explore the area.
If you're up for some off-the-beaten-tourist-track exploring I suggest picking up a copy of "Back Roads Ireland" by DK publishing. Most guidebooks (including the RS guide) don't really cover Donegal or Connemara in any detail (if at all), but the Back Roads guide lists a number of scenic drives and out of the way places that might be of interest and which might help you put together an itinerary for exploring the area. Can usually find used copies for just a few dollars on Amazon.
Might add that including a visit to Northern Ireland would be time well spent.

Thanks Robert. I will try to hunt down the book. We would definitely be driving, probably small van as we did last time in Ireland.

We stopped for 1 night on our way from N. Ireland to Galway in April and stayed at the Slieve League B&B. It was a short uphill walk to the cliffs and we passed only a couple people and lots of sheep. It was beautiful. If we'd had more time, there is a boat trip from Killibegs that takes you to the base of the cliffs which sounded nice.

Thanks Carol. Sounds brilliant.