My 2-cents worth (which might only be worth 1 cent as I've not been to Killarney): we were on a bike trip in 2011 and didn't get to Killarney but spent several days in and around Dingle. You didn't mention what time of year you'll be visiting, and it stays light quite late in the summer, but If you're not staying in Dingle for a night, then your original thought about doing a day trip to and from Killarney would at least break up your overall driving on those days into shorter segments. If you wait until your last day, then checked out of your room in Killarney and drove all the way to Dingle, spent the day there, then had to get all the way to Shannon (even 80 or 90 miles is much more wearing than it is at home, being on narrow roads on the left side of the road, driving from the right side of the car) even if you don't do the Conor Pass route, late at night after a long day in Dingle, that would make for a very long day/drive. You could enjoy Monday in Dingle, whereas waiting until Tuesday might cause some worry about getting to your airport room in time to get a few hours' sleep after spending the whole day (and a good part of the evening?) in Dingle. Have a great trip in any event!