Blathmac mac Con Brettan was an Irish fíle (poet) and monk whose floruit was around 760.
Blathmac was the son of Cú Brettan mac Congussa (died 740), seemingly a king of the Airthir, one of the Airgíalla kingdoms, situated in modern-day County Monaghan. His brother Donn Bó was killed in battle in 759. Cú Brettan and Donn Bó both appear as characters in the saga Cath Almaine and are portrayed as poets.
Blathmac was educated in a monastic school and went on to become a monk, strongly influenced by the ascetic Céli Dé (Culdee) movement. A manuscript containing his surviving poems, two meditations on the Virgin Mary, Tair cucum a Maire boid and A Maire, a grian ar clainde, once in the possession of Mícheál Ó Cléirigh, is in the National Library of Ireland, where it was re-discovered by historian James Carney.