and in what language. Research into the name O'Drennan revealed many variations, including 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