Thankfully local genealogist and historian Brian Mitchell has the answer having sorted the Dohertys from the McLaughlins, and the Gallaghers from the Kellys, in his book ‘The Top 300 Surnames of Derry-Londonderry’, published by Clearfield in Baltimore in the United States.
The book is based on the 1989 Foyle Community Directory, which lists 1,860 unique surnames in Derry City.
Brian looked at the number of entries and ranked the surnames in order of mentions. Where is yours in the list?

33. 7. Harkin.
7. Harkin. Irish: This County Donegal sept, with their homeland in the Inishowen peninsula, were hereditary tenants of the church lands of Clonca, near Malin Head (Brian Mitchell). Harkin ranks seventh on the list with 114 mentions. Pictured, on right, is former Northern Ireland striker Terry Harkin who helped bring senior football back to Derry in the 1980s with Barry McCreadie, Ivan Parke and Liam Coyle. Photo: Hugh Gallagher.

34. 6. Coyle.
6. Coyle. Irish: Meaning son of the servant of Comgall this sept established itself in Mevagh parish, County Donegal. An early form of the name was McIlhoyle. Furthermore Coyle has become confused with McCool (Brian Mitchell). Coyle ranks sixth on the list with 116 mentions. Pictured is Derry pop singer Nadine Coyle. Photo: Archive

35. 5. Moore.
5. Moore. Can be of English, Scottish or Irish origin. In England and Scotland the name derived from the personal name More, meaning Moor (i.e. the Muslim people of north west Africa) or from a local name for someone who lived on or near moorland. Moore became widespread as a surname throughout England and Scotland. In Scotland the name was also known as More and Muir. The Mores were a sept of Clan Leslie while the Muirs were a sept of Clan Campbell. In Ireland, the O'Mores were the leading sept of the Seven Septs of Leix (Brian Mitchell). Moore ranks fifth on the list with 125 mentions. Pictured is Children in Crossfire's Richard Moore with the Dalai Lama. Photo: Archive

36. 4. Kelly.
4. Kelly. Irish: At least seven distinct septs of the name established themselves in Ireland, the most powerful of which ruled over a territory which included east Galway and south Roscommon. In Ulster, a Kelly sept, claiming descent from Colla, the 4th century King of Ulster, was based in south Derry. Kelly was known as a surname in Scotland long before the 19th century immigration really established the name there; there was a Kelly sept attached to Clan Donald (Brian Mitchell). Kelly ranks fourth on the list with 142 mentions. Pictured is legendary Derry City outside left Jimmy Kelly. Photo: Archive

