of the eleventh century married the joint heiress of the Cineal Comhgall (after whom Cowall is named) and their collateral kinsmen the Cineal nGabrain of Knapdale. His two grandsons, Donnshleibhe (Dunsleve) and Domhnall (Donald) O Neill are the ancestors of the branches of the clan. From Dunsleve, lord of Knapdale in the early thirteenth century are descended the MacLachlans, Lamonts, MacSorleys, MacSweeneys, MacQueens or MacSweens and the MacEwens. The MacLachlans inhabited Strathlachlan in Argyle, and had their stronghold, Castle Lachlan, on the south shore of Loch Fyne. In 1230 the then chief Gilpatrick, son of Gilchrist (ancestor of the MacCilchrist branch of the family, lords of Glassary—see under Scrymgeour) witnessed a charter granted to Paisley Abbey by Laomainn, his cousin, ancestor of the Lamonts.

The Lamont territory was in Cowall, where they were the most powerful family until the great massacre of several hundred of their men, women and children by the Campbells in 1646, an act of revenge for the Lamonts’ complicity in the murder of several Campbells by MacDonnells from Antrim a few years earlier. After foolishly surrendering their castles of Toward and Ascog (on the southern extremity of the eastern and western peninsulas of Cowall, respectively) the garrisons, now at the mercy of the Campbells, were cruelly tortured and put to death, and the castles burnt and razed. The grandfather of Laomainn was the brother of Gilchrist, ancestor of the MacLachlans.

This grandfather, Ferchar, had two sons, Malcolm, father of Laomainn, and Duncan, ancestor of the MacSorleys (Mac Somhairle) of Glassary in West Cowall, the majority of whom later assumed what became the mutual clan-family name of Lamont. The Lyons of Glamis in the Strathmore district of Angus descend, according to tradition, from a scion of the Lamonts of Cowall. John the son of Lyon (Johannes fihius Leonis) and Hugo the son of Lyon (Hugo filius Leonis) were members of an inquest on the lands of Rostinot in 1321—1322. John Lyon had a charter of lands in Perthshire ca. 1342—43 from David II. Another John Lyon (or "Lyoun") appears, possibly the son of the former, as clerk and secretary to David II. He was known as the "White Lyon," which suggests an epithetic allusion to the "White Lyon on Blue" of the arms of the Lamonts, his own arms being a reversal of those colors. He was later granted the thanage of Glamis as a free barony by King Robert II ca. 1371—72, and soon afterwards married the king’s daughter. This family later became barons of Glamis (1445) and earls of Strathmore. Some small broken clans in Angus are recorded as petitioning to "be allowed to take the name of Lyon, and be counted clansmen of the Strathmores."