Brief biographies of two Irish saints of this name. The first mentioned was Archbishop of Armagh, and died in 851 or 852. The second, St. Diarmaid the Just, was a monastic founder and distinguished writer of the mid-sixth century.
Three or possibly four Irish saints of this name lived in the sixth and seventh centuries. St. Donnan of Eigg and St. Donnan of Auchterless are probably the same person. There was also a St. Donnan the Deacon, blood brother of St. Kieran of Clonmacnoise; and a St. Donnan who was a nephew of St. Senan and who succeeded St. Kieran of Clonmacnoise as Abbot of Aingin.