First a warning! When getting the bus to Kinsale from the Airport be bolshy! A queue formed when the bus pulled up at the airport. The person at the front of the queue asked if the bus was going to Cork and the driver replied (rightly) that it wasn't. However he then DROVE OFF! This was without checking if anyone else wanted to go to Kinsale. So we were left stranded at the airport and had to get a taxi which was 27 Euros. Not a nice start to our holiday.
Otherwise we found the buses pretty reliable if expensive. The bus runs Kinsale-Cork Airport-Cork City. The last bus back to Kinsale from Cork City is 10pm on week nights but around 7pm on weekends. It's about 45 minutes to Cork City from Kinsale and a return will set you back €9.40. You do not need correct change but they will not accept a €50 note. You can book tickets online and save 10% but then you need to use them on the day stated.
Timetables are available from the Bus Eireann website in PDF format. Overall Kinsale is not a bad place to go without a car.
The 249 to Kinsale bus leaves the Bus Station in Cork at regular intervals. Mondays to Fridays they are almost every hour, Saturdays almost every two hours and at 9.30, 12.30, 14.30 and 18.00 on Sundays. The return trips are at roughly the same intervals and details and times of all journeys are posted on the door of the tourist office in Kinsale. The fare is EUR 8.60 for a return ticket and you can buy your ticket from the machine. On Sundays from June 18th to August 13th (excluding August 5and 6) the 14.30 bus goes on to the beach at Garretstown outside Kinsale.
If you do go to Kinsale on the bus and you want to go to Charles Fort, I would suggest asking the bus driver to let you out at the stop before Kinsale and you can get there easily in 20 minutes walking. This means you get to see the fort first and can enjoy a leisurely walk into Kinsale after some refreshments, whereas if you go into the town and have to walk all the way up to Charlesfort and back again, that really is quite a trek.