Edward Kneafsey (1) in his book Surnames of Ireland highlights the "lack of intermingling" between the Cartons and the MacCartans geographicly especially in the North of Ireland; the Kneafsey paradox. His entry for MacCartan is:
McCartan is from the Irish Mac Artain, son of Artan, a diminutive of Art. Artan was a grandson of Montan Mac Guinness of Iveagh, in Co Down. The McCartans were chiefs of Kinelarty in the central par of Co Down. They were tributary to the O’Neills. Some families have lost the “Mac” and appear in English as Carton and Cartin, with a few Cartens and Cartans.
Some Cartons have a different origin. Those of the Midlands may be of O’Cartain origin. Those of Dublin and Wexford also did not have a “Mac”, the Dublin Families having been Hugenots.
The Map shows the distribution of some 500 families with telephones in 1992. Allowing for homes without telephones, there are probably 760 families altogether. Some 605 have the “Mc” prefix. They live in Co Down and south Antrim. In Cavan and Leitrim, the name is spelt McCartin. Carton and Cartin families, with some other variants, live mainly in the north of Tyrone, Derry and Antrim, and in the Republic.
The distribution shows the historic McCartan connection with central Co Down. About half of all the McCartan families in Ireland live in the county. Many of the remainder live in the now urbanised area of south Antrim.
The Carton distribution is quite distinct. Separate clusters are apparent in Dublin, Wexford, the Midlands and the North. The striking feature of the North is the lack of intermingling with McCartan. The loss of the “Mac” prefix appears to have been a regional phenomenon. The heartland in south-eastern Ulster has retained the prefix, whilst families in the north-western Ulster have dropped it. Northern Ireland had a third of all Irish families. It has 73% of the McCartans and 29% of the Cartons.
This is usefully called the Kneafsey paradox in the context of the study of the Cartin and MacCartan surnames. In his research Sean McCartan has come to similar conclusions.
1 Kneafsey, Edward Surnames of Ireland ISBN 0 940134 97 7 See also 179 McConville on Surnames in Ireland Extract http://www.mcconville.org/main/genealogy/surnames.html