I came upon an excellent & eloquent explanation of the usage of family names written by Blair MacAulay, Toronto, who is an authority on the genealogy of North Uist.
Here are the key points, to which I have added a few brief comments:
In ... the Outer Hebrides before about the year 1800 surnames did not exist!
A fact that simply cannot be over-emphasised...
For example, the tailor Angus MacPherson might be known as “Angus Tailor”. More frequently the “sloinneadh” was the person’s patronymic (e.g. “Aonaghus Iain Domhnullach” (Angus John son of Donald) which was the patronymic of Angus John MacDonald of Knockline, the well-known North Uist genealogist born in 1900).
Another example would be “”Domhnull mac Alasdair ‘ic Raonuill” (Donald son of Alexander the son of Ronald) (in Gaelic “mac” means “son” and mhic, or abbreviated “’ic”, means “son of the son”.
This partly explains why some 70% of the population of North Uist today has the surname “MacDonald”.
The attack upon Gaelic culture included every aspect of it, especially the language.
This is extremely important to understand for those attempting to research their own family history, and it didn't stop with the peoples names. Placenames suffered this same mangling in their Anglicisation too.
Over time certain Gaelic names came to have an “accepted” English equivalent, often with no obvious connection to the Gaelic name. For example a person locally known in Gaelic as “Gilleasbuig Mac Dhomnull” would probably appear in the Census or in a register of marriages etc. as “Archibald MacDonald”. Thus an official record may contain reference to a person under a name that was completely different to the name that he was known by to his contemporaries.
Were I to travel back in time & present my (painstakingly recreated) family tree to my ancestors, they would probably wonder who on Earth I was referring to!
With many thanks to Blair MacAulay for permission to quote these extracts.