Cattle Lords & Clansmen : The Social Structure of Early Ireland. Nerys Patterson, London, University of Notre Dame Press, 1994.
ISBN 0-268-00800-0
A wonderful study of rank and social structure in pre-Medieval Ireland. Draws heavily on Brehon law and has sections on rank, the structure and function of the clan group (geilfine), and how societal relationships changed seasonally. Not the easiest read in the world, but one of the best studies of early Irish kinship I've ever come across.