There is a surname McNiff, which appears to have its origin in Galloway, South-West Scotland. There the surname McNeill sometimes became McGreal, McNaughton became McCracken. See where this is leading? McNiff to McGriff - why not? It's a possibility. McNiff, according to Black (Surnames of Scotland), is probably the same as the Irish MacCondhuibh, englished as McAniff, Coniff and Cuniff. Condhuibh is the genitive form of Cudubh, meaning "black hound".