Want to have King Arthur descended from Fergus mac Roach/Mor? Her it is.
There were several Arthurs/Arthruis/Arthwys in the 5th and 6th centuries, and one can usually find some modern work that makes each of these the 'real' King Arthur. The only early evidence we have for a real King Arthur is in Nennius' 'heap' which itself isn't entirely trustworthy. However, the following account explains all of that away in order to make Arthur one of the Dalraida Scots, a descendent of our Fergus mac Roch/Mor. At the first click below you will see a genealogy of Arthur from the kings of Dalriada, where the first is the son of Fergus, as can be seen in an extend genealogy at the second click.