Dunoon is tricky to get to at times, hence why tourism has not blossomed so much here, but it is definitely worth it in the end!
By bus
Scottish CityLink runs bus services from Glasgow's Buchanan Street Bus Station roughly every hour through the day till evening to both ferry terminals. The bus ride takes just over an hour. Notify the driver of which ferry terminal you wish to be dropped off at. McGills buses also run a direct bus service, the Cowal Connector, from Glasgow to Dunoon, which stays on the Western Ferries ferry. It runs every few hours or so, check the timetable for details.
Train
Local First Scotrail trains leave Glasgow Central which is well connected to the whole of Britain for Gourock frequently. Buy tickets before boarding as there are manned ticket barriers. The journey takes 40 minutes. The last stop on this line is Gourock and the CalMac ferry terminal is located next to the station.
Car
By car it is simple using roads out of Glasgow. Travel west on the M8 motorway in the direction of Greenock and Gourock. The motorway eventually ends and becomes the A8 highway.It takes about 1 hour to get from Glasgow to Gourock. There are two ferry companies; Western Ferries and Caledonian MacBrayne Ferries. Western Ferries is clearly marked once you leave Gourock on the seafront and has big red ferries. Caledonian MacBrayne, or CalMac for short, has their ferry terminal in the middle of Gourock next to the railway station approximately 2 miles before Western Ferries terminal. Of the two ferries, Western Ferries has better frequency every 15 mins at peak timesvs CalMac which offers service hourly. Another difference is that CalMac has their Dunoon terminal in the main part of the downtown area while Western Ferries is out of the main part of town midway between Dunoon and Sandbank. Both crossings take about 20 minutes.