Yes, please don't be afraid to rent a car. I rented one last year, first time driving on the left, and even with a stick shift it was an east thing to do. Try to get the smallest car you can, it will be easier getting around, but as Bryno said, the drive out of the airport and to Skibbereen should not be a busy one and will get you all the practice you need.
And we stayed at the Castle Townsend last year also, wonderful place, but you will need a car to get around out there. They don't have any public transport to get you around, and they have car parking at the Castle.
Don't forget to have a pint and/or lunch/dinner at Mary Ann's, which is on the road down to the Castle, lovely place and great food.
Strange thing in the town tho, there is a tree right in the middle of the road! You will have to drive around it to get down to the Castle!
I have some pictures up that may interest you, and get you aquainted with the Castle area
Say hi to Tinker the cat if he's still around ;-)