(assuming you're staying in or around Dunoon)
This is a very enjoyable drive or bike-ride, 26 miles there and back. Or you can take a bus from Dunoon as far as Castle Toward (there are several busses a day) and then walk. From Castle Toward to Inverchaolain is roughly five and a half miles each way.
The trip is almost entirely along the lochside (Firth and then Loch Striven), so pretty flat most of the way, and there’s usually very little traffic (the road dead ends shortly after Inverchaolain). The coastline is beautiful, and there are lots of birds and animals to see. Here’s a map centred on Castle Toward, which is roughly the half-way point of the journey from Dunoon to Inverchaolain.
There is another way of getting to Inverchaolain from Dunoon – an attractive cross-country route of nearly six miles known as ‘the coffin route’. This has been more thoroughly described as part of the South East Cowal pilgrim route from Ardnadam to Inverchaolain. This one is not suitable for cycling unless you wish to carry your bike, as the coffin trail involves some off-road hiking. You may need your Ordnance Survey map for this, and make sure to wear good shoes and waterproofs. Here’s a map of the route, centred at the bottom of the Glenkin forestry track.
Follow the track SSW up the glen, past the Glenkin farm (now a bunkhouse) to the head of the glen. The track will eventually turn and head back down the glen, but we keep going SW towards the pass ahead of us, Bealach na Srèine (‘pass of the bridle’, or ‘of the stripe’). At the head of the pass (NS1177) we look down into Inverchaolain Glen and head down the hill, keeping Inverchaolain Burn to our left when we meet it. Half way down we come to the forestry track on the right (NW) side of the burn. Follow that down to the farm of Stronyaraig (Sròn Dhearg ‘red nose or promontory’) and then down to the shore road. Turning left we shortly arrive at Inverchaolain Church.
You could walk back the way you came, over the hill. Or you could take the shore road south to Castle Toward (five and a half miles) and catch the bus back to Dunoon. Check the Ardentinny-Toward bus timetables before you go!