Motto (where available) Thornton Early Origins Spelling variations of this family name include: Drennan, O'Drennan, Drenan, O'Drenan, Thornton and many more. First found in County Galway , and County Clare where O'Drennan was chief of Slieve Eise, Finn, and of Cinel-Seudna, a district on the borders of both counties.   More