Agreed. The problem is that there are too many plausible alternatives.
Lord Lyon Sir Thomas Innes of Learney in 1970 endorsed the view that "The first of the O'Beolan Earls of Ross was Fearcher MacinTagart, grandson of Gillianrias, and son of the Sagart, or priest, who was the co-arb, or hereditary abbot of the old monastery founded by the Irish St. Maelrubha at Applecross in the seventh century." (page 285)
* https://books.google.com/books?id=_U0Ii-Om3EwC&pg=PA341&lpg...
This is the theory Duncan was arguing against. Duncan might be right but I think we have to take seriously any statement by a man who was the chief herald for Scotland, particularly when he says this is "the other and more probable account, which the best authorities have adopted."
The case for this scenario is fleshed out by the Mackenzie history, which I also posted yesterday. The short version is that Fearchar was probably the son (or grandson) of Gilleanrias. The Ross Clan is called in Gaelic, Clan Anrias or Clan Gilleanrias (the descendants of Anrias or Gilleanrias). This was probably the Gilleanrias who was one of the earls who besieged Malcolm II in 1160. In a genealogy dated 1450 the chief of Clan Gilleanrias is Paul MacTire. This same Paul MacTire was called cousin in a charter from William, last O'Beolan Earl of Ross. The genealogy makes this Paul MacTire the 2nd great grandson of Gilleanrias, and takes the family further back to Gilleoin na h-Airde and beyond.
https://books.google.com/books?id=C9sGAAAAYAAJ&pg=PA44&lpg=...
The basic idea is that when the direct male line of the O'Beolan earls of Ross died out, the earldom passed to he Leslies, but the chiefship of the Ross clan passed to Paul MacTire, and since clan Ross was called Clan Gilleanrias, both the O'Beolan earls and Paul MacTire must be descendants of the same Gilleanrias.