There are only three feasible options
- use an organized tour
- take a taxi
- hire a car
It’s just not practical by public transport as it requires a train followed by a bus and takes around 2 hours.
Hiring a car is likely to be the cheapest option.
A taxi is the best option, 120 to 150.
It is about half the price of a coach tour and car hire is impractical for a short time.
You could just cut Kinsale and spend some time in Cobh as well as taking a train into Cork. While I encourage anyone to find out for themselves if they like a place, and I admit to not being a massive fan of Kinsale, I don't think it's worth paying more than about €20 each for a visit of a couple of hours.