McCullough is an Irish surname, with two distinct Gaelic origins: Derivation from the Irish Gaelic Mac Cú Uladh 'son of the hound of Ulster' is more probable.
[3] While Cú Uladh may allude to the legendary Irish figure Cú Chulainn, it was a common given name in medieval Ireland.
In Ulster it was often in use by the O'Neills of Clandeboye, the MacMahons of Oriel, the MacCanns, and the MacDonlevy kings of Ulaid.
The M[a]cCulloch spelling is more common in Scotland (especially Galloway), and is associated with Clan MacCulloch.