It is pronounced "oh-dee" (IPA: /oːˈdiː/) and is an anglicisation of the original Irish language name, Ó Deághaidh, meaning "descended from Deághaidh", the name of a tenth century clan chieftain.
The traditional Ó Deághaidh clan were based in the townland of Dysert O'Dea in County Clare in Ireland, near the modern town of Corofin. The important Battle of Dysert O'Dea took place there in 1318 when local clans of the Kingdom of Thomond, including the O'Deas, fought the Anglo-Normans.
The Irish alliance routed the Normans and, during the combat, local chieftain Conor Ó Deághaidh killed the Norman leader Richard de Clare with an axe. Thomond remained free of foreign rule for 250 years after this decisive battle until re-conquered by the English in 1570.
Diarmaid Ó Deághaidh built a castle at Dysert in 1470 and it is a museum today along with an an important Celtic high cross and an adjacent ruined monastery which was founded in the eighth century.