CAMPBELL  Mac Cathmhaoil  (cathmhaoil, battle chief).  An Irish sept in Tyrone; in Donegal it is usually of Scottish galloglass origin, viz. Mac Ailin a branch of the clan Campbell (whose name is from cam béal, crooked mouth).  Many Campbells are more recent Scottish immigrants.  The has been abbreviated to Camp and even Kemp in Co. Cavan.

Campbell is a Scottish surname, one of the ten most numerous in that country, and one of the thirty most numerous in Ireland, with over two-thirds of thos who bear the name living in Ulster.  It is particularly common in counties Armagh, Down and Antrim.  Originally a nickname, it comes from the Scots Gaelic cam beul, meaning 'crooked mouth'.

Clan Campbell was found by Gillespie Ó Duibhne, who lived in the thirteenth century, and was the first to assume the surname.  His descendants included the most famous branch, the Campbells of Argyll, one of whose members were responsible for the massacre of MacDonalds of Glencoe, which lead to the famous feud between the two clans.

The vast majority of Irish Campbells are descended from the Scottish family, although in County Tyrone the surname may be an anglicisation of the Irish Mac Cathmhaoil, from Cathmhaol, meaning 'battle-champion'.

MacCawell is given as MacCampbell in the Hearth Money Rolls for Co. Tyrone (1665). The Tyrone "Campbells" are usually not the Scottish Campbells (Mac Cailein) but the native Tir Eoghain sept, Mac Cathmhaoil. 'The sept MacCathmhaoil got their name from Cathmhaol, descended from Feradhach son of Muireadhach son of Eoghan (son of Niall Naoighiallach) As the MacCathmhaoils were the leading sept of Cenél Fearadhaigh, they are often called Cenél Fearadhaigh,sometimes Cenél Fearadhaigh Theas, to distinguish them from the offshoots of Cenél Fearadhaigh who remained in Inishowen or thereabouts. The MacCathmhaol sept were fixed in the Clogher area of Co Tyrone. It was their function to hold as a bastion for Cenél Eoghain against Cenél Conaill on the northwest and the descendants of the Colla on the south-west and south. Later they became an important church family. They receive mention in Ceart Ui Néil being, along with MacMurchaidh and O'Devlin, classed as "fircheithearna" (i.e. "true kerns") of O'Neill. Their importance is obvious from a glance at the events listed in connexion with them under MacCathmhail in the index to the Annals of Ulster. MacCawell is a rare surname in Co. Tyrone to-day, though the county has many Quinns, Devlins, Hagans, Donnellys, Gormleys and the rest of the Cenél Eoghain septs. There are quite a number of "Campbells" in Tyrone, and although they are officially "Campbells" the local colloquial name is "MacCawell." It is accepted that the rarity of this once important surname is simply that its bearers adopted the similar sounding Scottish name of Campbell. 'There are Mac Caileins in Tyrone as well, but not many. A particular Campbell might easily be of Scottish or Donegal origin, but Mac Cathmhaoil was one of the seven powerful septs supporting Cenél Eoghain and that sept alone could not virtually disappear while the other surnames continued to flourish.

Source:  Goireland.com website