There are two origins for the Irish surname McCullough. The majority of those who bear the name descend from the sept of Mac Cu Uladh or Mac Con Uladh. This this Irish surname is an Anglicized form of the names Mac Cu Uladh or Mac Con Uladh, both forms being used in Irish and both signifying “son of the hound of Ulster”. The Gaelic form of the name has also been Anglicized as Mac Cullach and Mac Culloch. The majority of bearers of this name are to found in Ulster, in particular in counties Antrim, Down, and Tyrone. One of the earliest records of the name is a reference to one Seamus Mac Con Uladh who was killed in Dunbo in 1532. The Composition Book of Connacht (1585) records one Rory Mac Uladh Mac Cullogh of Co. Sligo.

In some instances, the McCulloughs of Ireland may be of Scots Gaelic origin, being descendants of Scottish McCulloughs who settled in Ireland in the seventeenth century. In fact, they settled in Ulster which has of course led to some confusion as to the origins of the name in that province. The Scottish MacCulloughs derive their name from the Gaelic word “culach”, meaning a boar. The earliest record of the surname in Scotland dates to the fourteenth century. The most common spelling of McCullough in Scotland was MacCulloch. The first McCullough listed was the King of Scotland at the time of MacBeth.