This paper is concerned with two distinct but related topics: the origins and rise of the MacCarthy dynasty in pre-Invasion Munster, and the political geography of the 12th century MacCarthy kingdom of Desmuma or Desmond (south Munster). The period covered by both studies is approximately AD930¿1200. The MacCarthys¿ original power-base was the ecclesiastical city of Emly, Co. Tipperary, and subsequent centres of power included Cashel and Cork, for the MacCarthy kingdom of Eóghanacht Chaisil represented the only significant opposition to O Brien hegemony in Munster. The fluid political position here in the 11th century hardened into the typical division of Munster into northern (O Brien) and southern (MacCarthy) segments in the 12th century, although some fluidity of borders remained. The territory of Desmond is explored and its constituent kingdoms and regnal lines examined. At various stages Desmond contained all of the modern counties of Cork, Waterford and Kerry, significant portions of that of Limerick, and parts of southern Tipperary, and this study has relevance for the medieval history of all of these areas in this period.