Uí Díarmata was a local kingdom located in what is now north County Galway.
The ruling dynasty took its name from King Diarmait Finn of Connacht (died 833), and the territory in turn was named after them. It seems to have been created by the Uí Briúin in the ninth century during a wave of expansion under his grandson, Uatu ua Diarmada. Its kings appeared regularly in the annals from 971 onwards
By the 11th century its kings had taken the surname Ó Con Cheanain (Concannon). The Annals of Connacht state that "Domnall son of Aed O Con Chenainn, king of the Uí Diarmata, and Muirchertach his brother" were killed at the Second Battle of Athenry in 1316. By this stage it had become incorporated into the territory of Clantaie O Dermod (Clann Taidg and Uí Díarmata) ruled by the de Berminghams, Baron Athenry.
Writing at Tuam on 13 September 1838, John O'Donovan wrote "Henry O'Concanon Esq. of Waterloo near Glentaun in the parish of Killascobe in the Barony of Tiaquin enjoys hereditary property, and is the reputed head of this once respectable family." (2009, p. 35)
From the Annals of the Four Masters: