The Déisi were a class of peoples in ancient and medieval Ireland. The term is Old Irish, and derives from the word déis, meaning "vassal" or "subject"; in its original sense, it designated groups who were vassals or rent-payers to a landowner. Later, it became a proper name for certain septs and their own subjects throughout Ireland. The various peoples listed under the heading déis shared the same status in Gaelic Ireland, and had little or no actual kinship, though they were often thought of as genetically related. Déisi groups included the Déisi Muman (the Déisi of Munster), Déisi Temro (of Tara), Déisi Becc (located in the Kingdom of Mide) and the Déisi Tuisceart (the Northern Déisi; a sept of which would become famous as the Dál gCais). During the Early Middle Ages some Déisi groups and subgroups exerted great political influence in various parts of Ireland, and certain written sources suggest a connection to Britain as well.

The Déisi were a class of peoples in ancient and medieval Ireland. The term is Old Irish, and derives from the word déis, meaning "vassal" or "subject"; in its original sense, it designated groups who were vassals or rent-payers to a landowner. Later, it became a proper name for certain septs and their own subjects throughout Ireland. The various peoples listed under the heading déis shared the same status in Gaelic Ireland, and had little or no actual kinship, though they were often thought of as genetically related. Déisi groups included the Déisi Muman (the Déisi of Munster), Déisi Temro (of Tara), Déisi Becc (located in the Kingdom of Mide) and the Déisi Tuisceart (the Northern Déisi; a sept of which would become famous as the Dál gCais). During the Early Middle Ages some Déisi groups and subgroups exerted great political influence in various parts of Ireland, and certain written sources suggest a connection to Britain as well.