MacCullagh and MacCullough are very numerous in Ulster, and almost entirely confined to that province. They may have a native Irish, or a Scottish origin. The Irish original is Mac Con Ulaidh or Mac Cu Ulaidh, both meaning "son of the hound of Ulster",and generally anglicised as MacCullagh, though they also sometimes appear as Coloo, MacAnaul and MacNully. The family were based in east Co. Antrim and north Down. The Scottish origins are various, coming from Mac Cullaich, from cullach, meaning "boar", common in Galloway, and from Mac Lulich, (lulaogh meaning "little calf"), which originated in Argyllshire. A member of this latter family was briefly King of Scots. In Scotland, the surname is generally given today as MacCulloch. In Ulster MacCullagh is more common in the west of the province, in particular in Co. Tyrone, while MacCullough appears more frequently in the east, in counties Antrim and Down.

McCullagh households by county in the Primary Valuation property survey of 1848-64.
Click on a county name for a parish breakdown(paying).
Antrim 143
Armagh 77
Belfast city 28
Cavan 26
Cork 9
Cork city 3
Derry 67
Donegal 32
Down 248
Dublin 8
Dublin city 13
Fermanagh 15
Galway 5
Kildare 2
Kilkenny 3
Laois 1
Leitrim 8
Limerick 1
Louth 43
Mayo 1
Meath 17
Monaghan 41
Roscommon 3
Sligo 11
Tyrone 319
Westmeath 10
Wexford 2