A Brief History 

Excerpt from Wikipedia

Clan Ewen of Otter claimed descent from Donnsleibhe, who was said to be a descendant of an Irish prince of the O'Neill dynasty named Ánrothán Ua Néill, who left Ireland for Kintyre in the 11th century. He was a son of Áed, son of Flaithbertach Ua NéillKing of Ailech and Cenél nEógain, died 1036. There are several other Argyll clans which claim a descent from this prince—Clan LamontClan MaclachlanClan MacNeil of Barra, and also the MacSweens who left Scotland to settle in Ireland in the 14th century. From this descent, these clans claim a further descent from the legendary Niall NoigíallachHigh King of Ireland, who lived from the mid 4th century to early 5th century.

The only genealogy to survive regarding Clan Ewen of Otter, is the so-called MS 1467, now held in the National Library of Scotland. The Gaelic manuscript was written in 1467 and contains the genealogies of many Scottish clans. Unfortunately the MacEwen genealogy is practically unreadable in places. The MS 1467 was uncovered by W.F. Skene in the early 19th century, who transcribed and translated it. The following is his translated transcription for the MacEwens of Otter:

The chiefs of the clan lived at Otter, on Loch Fyne. Their castle, 'MacEwen Castle' was located on the rocky shore of the loch, near Kilfinan. Ruins of the castle are still present in the area.

In March 1432, Swene MacEwen resigned his title to the Barony of Otter to his feudal lord, King James. The king restored Swene to his title, but designated Gillespie Campbell (heir to Duncan Campbell of Lochow) as heir to the Barony of Otter. When Swene died in 1493, the barony passed into the hands of the Campbells.

Since the death of Swene the line of chiefs of the MacEwens of Otter has been untraced, however according to tradition, a MacEwen clan arrived in the earldom of Lennox "under a chieftain of their own" during the fifteenth century; the same tradition, which refers to a new banner, suggests that the clan chief was granted arms by Mary Queen of Scots at some time before her defeat at The Battle of Langside in 1568.

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MS 1467