 holding lands in "Ros Buill" (old kingdom of Ross Guill) now encompassing part of County Donegal.The translation of the "Ceart" is found in Studia Celtica.John O'Hart also gives two forms of the modern MacTavish, as well as an old form in "Irish Pedigrees, The Origin and Stem of the Irish Nation"

Another source for an Irish MacTavish origin is the "Topographical Poems of O’Dubhagain", c. 1372, that illustrates what the origin of the MacTavishes appears to be.