If you're coming from Seattle, you first evening will be a fog, at best. You'll be leaving Dublin in the morning to get to Killarney at a reasonable hour, and don't forget all the stops along the way that are also "can't be missed". You'll have practically no time in Dublin. Dingle is a great place to visit, but it is one of many great places on the island. We're fans of trad sessions, so the evenings/pubs are the main draws to Dingle (and anywhere) for us, so you miss all that with a day trip. We often use one night stays, because the things we want to see are between towns - two nights means backtracking. We try to plan our stops so that we don't spend all day in the car - maybe two hours of driving with three or four one hour stops at interesting places along the way plus lunch. Driving is slower than Google forecasts.
If you want specific advice, indicate what your group likes to do/see (hike, shop, castles, Neolithics, music, scenery, museums etc.), if it's your first trip, other wrong side driving experience, arrival/departure times.