The most direct route (if you can call it that!) for me from Cork/Kinsale would be viaKinsale - Bandon - Dunmanway - Glengarriff - CTB. There are a number of clever 'sub-routes' to get you there, but I doubt your sat nav will have enough Gigabytes to calculate those ones, so best to stick with that route if you can. This journey is 76 miles and will take you three hours. I can hear your guffaws already !!!! However the route is not direct and as you must navigate your way through a number of towns and villages on the way, you must resign yourself to 3 hours + in the car. However, don't be too disappointed as the journey is very pleasant, interesting, and you will get to see a lot of beautiful west Cork on route. But as you see, even by leaving Kinsale at 9am, you will have plenty of time to get to CTB leisurely by 3pm.
You did mention driving the coast road and yes, this is actually the route I would suggest to you (my previous paragraph was simply a ruse to suck you in and get you on board !!!
This journey in fact is only 20 miles longer but in terms of time will not take you any longer than the previous route mentioned.
If you go the Coastal route i.e Kinsale -Timoleague - Clonakilty - Skibbereen - Bantry - Glengarrif - CTB, it will give you an opportunity to take little side trips to Timoleague, Courtmacsherry, Glandore (for lunch maybe?) or Castletownshend (I recommend a visit to the historical St. Barrahane's there and lunch at Annies Pub), Heritage Centre at Skibbereen, (home to the Great Famine exhib.) Lough Hyne, Bantry House etc.
With stops, this journey could in fact take you 5 - 6 hours, but you have that amount of time so it is recommended.
As you pass Glengarrif (whether which way), you will see signs for trips to Garnish Island. This is a must do, but not on your day of travel, keep it in mind for one of your subsequent days in Castletownbere. Then, choose a pleasant day and do the Garnish Island trip. The €12 + €4 boat trip and entry fee seem a little steep until you get out there and then you realise how much it is worth it. Bring a picnic, spend a few hours there. It's very nice.
As for Beara Peninsula, well that is another story and if you have time, then a day trip doing that circuit is highly recommended. Just load up that camera of yours with Gigabytes of memory and take an oxegen tank for all that breath of yours that will surely be taken away ;-))
