West Cork is wet — wetter and milder than most of Ireland, with rain possible in any month. May to early October is the sensible window for visitors, with the longest days, the warmest sea temperatures, and the best chance of the headlands appearing out of the mist. July and August are the busiest, but Bantry has never become a coach-tour town and even at peak season it doesn't feel crowded.
The two events worth timing a trip around are the West Cork Chamber Music Festival, held at Bantry House over ten days in late June and early July, and the Bantry Mussel Fair on the May bank holiday weekend. Both fill the town's accommodation, so book ahead.