My 3x G Grandfather was an Archibald "Guilleasbuig" MacArthur, born in Knock,Carlowoway in 1761. He was the son of Donald Macarthur, born in Carloway in 1730.
Donald may have been the son of an Angus Macarthur who was born at Bosta, Bernera, (but there are no records for this)!
Can't find a "Calum" relation anywhere though.
Regarding the origins of Clan Arthur, I did read a theory somewhere that they came from around Lennox, Strathclyde before moving to Argyll, and that they were descended from ancient Brythonic peoples. That would suggest they were related to the old Cornish and Welsh, as well as the Brythonic in Cumbria. But it is so far back in the mists of time, who really knows?
If you are on Gedmatch, my kit no. is A040435.