Who were the combatants at this famous battle? Two clans "Kay" and "Qwhewyl" are named in the historically reliable sources-- but who are they?

I would venture to suggest that "Clan Kay" is now generally identified as the Davidsons (Wyntoun's account names the clans involved in the battle as "Clachynnhe Qwhewyl, and "Clachinya" [Clann Dhaibhidh]), and Clan Qwhewyl is synonymous with "MacMillan". The original form of MacMillan in Gaelic is MacGhillemhaoil, and in the late 14th and early 15th century the following spellings occur: MacGilveil, MacIllywoul, and MacKevoil. Clan Ghille Mhaoil also appears as Clan Chewill, Clan Kevoil, and Clan Qwhewyl.

At this juncture it may be appropriate to look at the battle, and the motives of the king in having two clans slug it out-- to the death.

Like so many things in the Highlands the lead up to the battle was a feud between rival clans -- the Camerons and the Macintoshes-- over land in the area of Loch Arkaig and Glen Loy. Originally held by the Macintoshes, this land was seized by the chief of the Camerons, and settled on John, son of Malcolm Mor, chief of the MacMillans. The Macintosh demanded rent, Lochiel refused, and things went from bad to bloody. Finally things reached a head and king Robert III decided to resolve matters once and for all by calling on the two feuding clans to settle matters by force of arms. Whether the king actually wanted to resolve the dispute, or merely wanted to see the two clans exterminate each other will probably never be known. In any event neither chief took the bait. Instead each chief sent his own representatives: thirty Davidsons on behalf of the Macintosh, and a like number of MacMillans to fight for Lochiel. Again, according to Wyntoun, a total of fifty men were slain at that battle; Shaw Farquharson, chief of the Davidsons escaped with his life by vaulting the barrier and swimming across the River Tay. Shortly after the battle Lochiel granted charters to a number of MacMillans for the lands previously in possession of the Macintosh (including a charter to John, and his son Gilchrist who led the clan at the Barrier Battle of Perth).

The Reverend Somerled MacMillan, in his book "The MacMillans and Their Septs", K and R Printers, Glasgow, 1952, goes into greater detail, and provides the necessary genealogical and historical references to establish both the Davidsons and MacMillans as the two clans who fought at the Barrier Battle of Perth.