You might be optimistic. It could take 1.5-2 hrs to get from Kinsale to Blarney, you've got to go through Cork city centre, I think, and it's a horrible city to drive around, all one way systems and traffic lights. I've never been to the castle, so I don't know how much time it takes to tour, but Blarney town itself is quite pleasant. I'm not sure you could do both places in a day, to be honest. Is there anything in Kinsale you're particularly looking for? I'd leave Kenmare and go directly to Blarney, then back into Cork for dinner and sleep.

At that hour of the morning, you shouldn't have any difficulty getting from town to the airport in less than half a hour, but I guess it depends on yourself whether you want to stay in town or be safe and stay near the airport.