Hamon, just checked Black's Surnames, a Gilcrist Makwrerdy held the lands of Bransar on Bute in 1506.
Mainly found on Bute & Arran and every instance quoted spelled different. Patrick MacVarthie, portioner of Stravanan, Bute in 1622.
The name occasionally cited in the Icelandic Sagas as Myrkjartan, the name of an Irish king. Modern Gaelic, M'Urardaigh. How many folk can boast such a pedigree?