As you can see there is so much to do in Killarney and it depends how much you want to cover. I would say, plan to spend 3-4 days in Killarney. You can definitely see the National Park and Ring of Kerry in 2 days but 3+ days will allow you to explore at a slower pace and really enjoy the beautiful landscape. Kerry is one of the most beautiful places in Ireland so be sure to dedicate enough of your time there. The biggest mistake I see travelers make when they go to Ireland is trying to pack to much in - aiming for quantity over quality. Ireland is a place to relax, unwind and enjoy nature. Take your time and enjoy where you do visit. The rest will be there on another trip.