[2] The death of his father Tommaltach mac Murgail (died 774) is recorded in the annals where he is called king of Mag nAi.
The annals state that his reign began after Battle of Sruth Cluana Argai (Cloonargid, Co. Roscommon) in 792 when he defeated Cináed mac Artgail (d. 792), of the Síl Cathail sept, who was slain.
Among the slain were Cathmug mac Flaithbertaig of the Cenél Coirpri and Cormac son of Dub dá Crích of the Uí Briúin Bréifne.
[8] The Annals of Innisfallen refer to Colla mac Fergusso (d. 796) as king at the time of his death, and he may have been a rival to Muirgius.
[12] Muirgius then turned his attention to the south and in 802 he destroyed the fortress at Loch Riach(Loughrea) in the Uí Maine territory of Maenmag.
[18] In 808 Muirgius gave support to Conchobar mac Donnchada (d. 833), the king of Mide of the Clann Cholmáin in his revolt against Aed Ordnide and advanced with the Connacht army as far as the assembly grounds of Tailtiu.
[21] Muirgius continued to seek support from the church and in 811 Nuado, abbot of Armagh visited Connacht with the casket of Saint Patrick and his Law.