New here - need help with understanding Clans

[O'Callaghan]

The above helps to explain the problem with Scottish septs. They are other surnames associated with a clan, but unless your family came from the same geographical area as those of the clan name, then you are not truly one of that sept, so therefore not one of the clan. This is because of the origins in taking up arms for defence. Many surnames have multiple geographic origins, so it is necessary to be careful over this. That said, if the older members of your family claim to be part of a specific clan, then they generally are.

I say Scottish septs, because in relation to Ireland the words clan and sept are said to be synonyms, and only those descended from the clan are seen as belonging to it. Most Irish do not belong to a clan, and Irish tartans are another conversation.

As for lowland Scots clans, there were a few even going back to early times. It is still true, however, that most lowlanders did not belong to one historically. One such border clan is Davidson, my wife's clan. Now, Davidson is not the Gaelic form of the name, but nobody in the borders spoke or speaks Gaelic. OTOH, there are many (most?) Davidsons having no connection to the clan whatsoever.

There are many names that are not clans, or even septs of a clan, that nevertheless have their own tartan. Suffice to say it doesn't take much to register a tartan. An example is Hart, which occurs in my own family tree, and which can be either English, Irish or Scottish (and in our case might be any of the above). Now, the tartan is Hart of Scotland, implying you have to be of Scots descent to wear it, and yet AFAIK there is no Hart clan, nor any Hart sept of any other clan. If you might think that were an unusual situation, you would be wrong. It is also true of several other tartans.