Just another option (so many) if you're coming down the west coast from Kilmartin. Come via Arran.

After leaving Kilmartin you could get the last boat (1900) from Claonaig (N Kintyre) over to Arran. Next day do Machrie Moor stone circles (superb) and anything else you fancy on Arran before leaving on the other ferry route to N Ayrshire (about 1.5 hours from the Dumfries and Galloway sites).

As others have said, the ferries are not cheap - at least this option does save a good bit of driving time.