Irish families as septs of Highland Clans

[O'Callaghan]

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 ?

The main migration of Gaels to Scotland from Ireland pre-dates surnames, so even where the name is similar or related there is often no link atall, but of course there were movements in both directions.

Kennedy is a particularly interesting one, as there are not merely two Kennedy clans, but three. As well as the Scots clan, and I suppose all Scottish Gaels were of the Dalriada, there were Irish Kennedys of the Dalcassians and of the Eugenians, these all being tribes of Gaels, for which I have given the anglicised versions of their names. Callahan is a Eugenian name, but the really famous Kennedy clan (think US politics) is Dalcassian, not Eugenian, so not of my tribe. The name Kennedy is simply the anglicised form of the first name Cenneidhe, of course, which I suppose must have been a popular name. Most Gaelic surnames were formed that way. Of course, not all the Scottish clans were Gaelic, whereas I think generally the Irish ones were, but that's another matter.

Not a historian or in any way qualified in what I am talking about, LOL!