Cairell mac Fiachnai (died 819) was a Dal Fiatach king of Ulaid, which is now Ulster, Ireland.
He was the son of Fiachnae mac Áedo Róin (died 789), a previous king.
In 809 Cairell challenged his brother Eochaid mac Fiachnai for the throne and defeated him in battle.
[2] This challenge had occurred after the high king Áed Oirdnide campaigned against Ulaid and ravaged from the Bann to Strangford Lough.
In 819 Muiredach mac Eochada (died 839) obtained revenge for his father by defeating and slaying Cairell in a skirmish in 819.