Would like to spend a week or so in this area. How easy is train or bus to use for this
There isn't much choice in buses from Kinsale.
Locallink have a 5x daily service to Clonakilty, which goes via Ballinspittle, Garrettstown, Timoleague and on some services Courtmacsherry. Garrettstown is a decent, but unspectacular beach. Timoleague has the abbey, which is worth seeing, while Courtmachserry is a pleasant seaside town with a good walk along the coast from the town.
Locallink also have a 4x daily service to Charles Fort, but I would recommend walking this if you are able as it's a pleasant walk 40 minute walk along the harbour (look for signs for Scilly Walk when you reach the Spaniard pub instead of walking on the main road).
For pretty much anywhere else you'll need to change in Cork