Must Sees are tough to counsel, such an individual taste. My two are in Clare: Dysert O'Dea and Corcomroe Abbey. Kerry is beautiful also, I found it more time consuming roadwise. Ross Castle and the Killarney NP are closer stops; the ROK and the Beara etc are harder on the time budget. There is so much to see, the farther into the beauty you go, the more time you spend. WC

If I understand it, you have about four days, but the first is the day that you arrive after a transatlantic flight, so that pretty limits your drive time, that day.

If it were ME, I think I would take the train to Killarney and then do day tours from there.  www.derostours.com offers daytrip coach tours of the Ring of Kerry, Dingle and the Gap of Dunloe, plus you would have access to the National Park, Muckross House, Gardens and Traditional Farms and Ross Castle.

Now, Killarney gets a bad rap as being 'Too Touristy', too crowded and a bit too much 'Stage Oirish', PARTICULARLY IN AUGUST, but the scenic beauty of the area, the abundance of available attractions and the direct train link to Dublin makes it an ideal choice for short breaks.

If the Cliffs of Moher and the Burren are more important, though, the same can ALSO be said for Galway.  Options there include the Connemarra, Aran Islands and the Cliffs.

Tough choices, but with only 3+ days, in August, probably your two best, unless you want to spend most of your time in a car  ...

Bob