Diarmait mac Conaing (died 786) was a King of Brega from the Uí Chonaing sept of Cnogba (Knowth) of the Síl nÁedo Sláine branch of the southern Ui Neill.
The high king Donnchad Midi (died 797) had campaigned against Leinster in 780 and then made peace.
The next year in 781 the Battle of the Rig was fought between the Síl nÁedo Sláine and the Uí Garrchon branch of the Laigin and their king Cú Chongalt was slain.
Diarmait and his cousin Conaing mac Dúngaile led the Uí Chonaing sept.
[2] The high king Donnchad Midi revenged this by attacking and defeating the forces of the Síl nÁedo Sláine at the Battle of Lia Finn or Tuilén and Diarmait was slain along with his kinsman Conaing mac Dúngaile[3] Both septs, the Uí Chernaig and the Uí Chonaing, again participated.