Disclaimer: I don't own a car, and it's been a while since I drove to Kinsale, but I did ask the guys at work who grew up here...

All of it really depends on the traffic. The word on the street is that it should take maybe 20 minutes to get from Kinsale to the airport, 40 into the city. In terms of Blarney, it's just like any rush hour: getting through the city could be tough. But any other time it's not such a big deal. And the road from Blarney to Cork is fine, multilane, etc. Maybe 20 minutes.

As far as coming from Kenmare, I don't see why not. If you're planning on travelling that day, then just head out early and go through Killarney. Taking the back roads through West Cork, while fun, can eat up a day.

Now the other thought is that while it could take a while to get to the airport... it's Cork. If you get up early and head out from Kinsale, my money is on making it there. More than a few people commute from Bandon or Macroom, so it's not completely unheard of. Then, you could see the castle first and Kinsale later, stay overnight, and just run up to the airport in the morning. Plus, then you get to relax in Kinsale and probably find a nicer place to stay than the airport.

Now, although Cork is certainly a rat's nest of one-ways, it's rather straightforward in terms of going from Blarney to Kinsale:
1. Follow the main road in from Blarney to the city centre
2. Once you cross the north channel of the river (near the Opera house) hang a left.
3. Keep going left until the port. It's a three-way intersection with Jury's hotel on the right. Turn right.
4. Follow that road until it makes a right turn, keep following that. At the next traffic lights, you should be at the South Link Road. Turn left there and head straight up to the Kinsale Road roundabout. The airport should be clearly marked from that, and Kinsale is on that road as well.