Bantry is at the end of the road from Cork — there is no train and no airport closer than Cork, which is about 90 minutes by car along the N71 through Bandon, Clonakilty and Skibbereen. Bus Éireann route 236 connects Cork city centre to Bantry several times a day; the same route continues on to Glengarriff and Castletownbere on the Beara Peninsula.
Driving down from Cork airport adds another twenty minutes. If you are coming from Killarney over the Healy Pass, leave a couple of extra hours for the views and the slow road.