Irish families as septs of Highland Clans

[TheMcManus]

I am puzzled by the number fiz-,kirk-, o' and the other common Irish names being listed as septs of Scottish Clans.
I know the obvious , the migration from Ireland brought many of those families over and the clan system integrated them at some point. Also I know that some clans are of Irish origin and that some families and clans share a common name but have different origins -like Kenedy.
Personally , I would like to understand the connection and relationship between the Catholic MacManus-MacMains and the Colquhon clan, other than the fact that the MacMains served the clan there isn't much info.
Gunn and MacManus-MacMains makes more sense because of the Norse-Gael lineage. But that those are questions that relate specifically to me , in general ..I cant find anything but general generic info on the relationship between the clans and the septs that have obvious Irish names.
Does any one have any insight ?