MacDonell of Antrim question

[Damion]

McDonnells didn't control Antrim, they controlled a smallish coastal strip made up of several glens. They were part of a larger region that included Islay where the Clan seat was and Kintyre, both in Scotland. Look up MacDonald of Dunnyveg. The biggest family in Antrim at the time were the O'Neills. One of the reasons the clan was at war so often is that the English tried to remove them directly and/or play the Irish against them because they didn't like having a region so close to their lands in Ireland that were subjects of the Scottish crown.
The McDonnells are not related to the O'Donnells of nearby Donegal who come from a Domhnall of the O'Neill.

It raises the question though why the Scots Gaels didn't use O'. The Irish used clann in a similar way but favoured O' over Mac so it seems there was some difference in culture. The O'Neill clan of Clannaboy for example: Clann Aodh Buidhe - clan of blond Hugh.