Although there is a good rail link between Glasgow and Oban we had a long but scenic drive from Aberdeen a journey which lasted five hours with some little toilet stops for the dogs, the whole journey covered over 200 miles. Some of the journey is on good roads mainly dual carriageways. After Perth you then have to travel cross county to the west on two laned roads. In and around Argyll we travelled on a lot of B listed roads and a lot of unclassified single track roads which means life very much in the slow lane. There are many off road parking places which most clearly state "No overnight parking" most are screened with hedging and trees. If you are lucky you will come across a few with open views - I was again delighted to spot some highland coos - this time with their new born cute calves just outside Connel. Public transport i.e. buses are rare and infrequent around these rural parts so you may want to do a self drive in your own or hired car around here. BTW the website I have included provides some great free maps to speed you on your way - but do take your time to see the splendid scenery which lies just at your break pedal - but don't just break pull off the road into a parking place!!
In order to reach Kilmartin you will need a car, unless you feel like waiting for the bus which runs once a day from Lochgilphead, or take the daily bus from Oban.
Kilmartin is very easy to find, if you're in Oban it's approximately 30 miles south on the A816, the road goes right through the village.