Muirgius (d. 815), son of Tommaltach and overking of Connacht, belonged to Uí Briúin Aí, a segment of Síl Muiredaig. This lineage, descended from Muiredach Muillethan (d. 702), who had contested the overkingship of Connacht with Cellach (qv) son of Rogallach, was located in the plain of Mag Aí (straddling north Co. Roscommon and north-east Co. Galway). Muirgius's father Tommaltach had died in 774 as local ruler of Mag Aí; his predecessor in the provincial kingship was his father's first cousin, Tipraite (d. 786) son of Tadc. It appears that Muirgius had at least two brothers, Diarmait and Fínshnechtae, and four sons: Cathal, Cormac, Tadc, and Flaithnia, but the names of neither his mother nor his wife are recorded.
Muirgius rose to prominence in the late 780s against a background of political chaos in which rival Uí Briúin lineages of Síl Cathail and Síl Cellaig, and the older dynasties of Uí Fhiachrach and Uí Ailello, were all in contest for provincial supremacy. Old sub-kingdoms of tribal origin, including Ciarraige Aí and Luigne, were drawn into the conflict in support of their overlords. If Muirgius did not actively exploit these conflicts, he certainly profited from them in no small measure. By 792 he was ready to face his Síl Cathail rivals directly; he defeated and slew Cináed son of Artgal, claimant to the overkingship of Connacht, at Sruth Cluana Argait (Cloonargid, Co. Roscommon). Shortly afterwards, an alliance of Síl Cathail and Uí Ailello was defeated with heavy loss at Ard Maic Rími (probably in the barony of Tirerrill, Co. Sligo). Significantly, it is from this year that the annals date the reign of Muirgius as provincial king, although the regnal lists imply that he succeeded his father's cousin directly. The death (796) of the leading Síl Cellaig dynast, Colla son of Fergus, removed Muirgius's most serious remaining rival from the scene.
In the years that followed Muirgius consolidated his position, moving against sub-kingdoms that had supported dynastic opponents. Even if the first of these ventures, an encounter in 796 with the Ciarraige Aí at Áth Féne in his own home-territory, proved embarrassingly unsuccessful (with Muirgius obliged to flee the field), he was ready to strike again three years later. In 799 he defeated what appears to have been an alliance of the Ciarraige and Uí Fhiachrach at Dún Gainiba. In 802 he sacked the crannóg of Loch Riach (Loughrea, Co. Galway), a stronghold of the Uí Maine dynasty bordering on the sub-kingdom of Sogain; it is tempting to see the hand of Muirgius in a revolt of the Sogain against their Uí Maine overlords the following year. By this time he was effectively master of the province. Periodically, he faced opposition from the older sub-kingdoms but was generally successful in suppressing resistance. Muirgius held the Ciarraige responsible for the murder of his son Cormac the abbot in 805, and he laid waste their territory. In 810 he avenged the slaying of his sons Tadc and Flaithnia by devastating the sub-kingdom of Luigne (barony of Leyney, Co. Sligo). Two years later, he reasserted his authority over the south of Connacht and was perhaps responsible for a slaughter of the Calraige of Mag Luirg, near Lough Arrow.
An important development in the reign of Muirgius was the beginning of Uí Briúin expansion into the Connacht–Ulster buffer zone of Bréifne; his conflict with the Calraige should perhaps be seen in this context. This expansion brought him into direct conflict with the political interests of the Cenél nÉogain king of Tara, Áed Oirdnide (qv). When the latter temporarily deposed (805) the overking of Leinster, Fínshnechtae Cetharderc (qv) son of Cellach (qv), it appears that Muirgius intervened. Certainly he actively supported a revolt (808) against the king of Tara by the Clann Cholmáin ruler Conchobar (qv) son of Donnchad Midi (qv). Muirgius hosted as far as the assembly-site of Tailtiu (Teltown, Co. Meath) and, even if the episode ended in ignominious flight, it is surely significant that the king of Tara attempted no punitive incursion into Connacht.
To further advance his political interests, Muirgius sought to align himself with the church at both local and inter-provincial levels. It seems that he imposed his son Cormac as abbot of Baslick, an old foundation in Mag Aí which had close associations with the Ciarraige. Presumably, local resentment led to the consequences outlined above. Early in his reign Muirgius had helped the cause of Roscommon by promulgating the ‘law’ of its founder-saint Comán in 793. Later on, he apparently sought the favour of Armagh; like his predecessor, he supported the Law of Patrick (qv) in 799 and again in 811. Nor did the expanding foundation of Clonmacnoise, long associated with the old tribes of Connacht, escape his attention. Muirgius seized his opportunity in 814, when the bishop of Echdruim (Aughrim, Co. Galway), a dependency of Clonmacnoise, was slain. The Uí Maine were blamed; Muirgius, with Abbot Fairchellach, led a punitive expedition into their territory, and many innocent people were slain. The abbot died shortly afterwards, but Muirgius promulgated the Law of St Ciarán (qv) throughout Connacht. This was his last recorded act as king; he died early in 815.
In the aftermath of his death, the provincial kingship was held for a time by a Síl Cathail dynast named Máel-cothaid, but Muirgius's brother Diarmait (d. 833) soon emerged to claim that dignity. Subsequent overkings included Muirgius's other brother, Fínshnechtae (d. 848), and his surviving son Cathal (d. 839). One of his grandsons, Conchobar, son of the Tadc who was slain in 810, died in 882 as overking of Connacht and was the ancestor of all the later Síl Muiredaig kings. From this grandson of Muirgius descended the dynastic family lines of Ua Conchobair (O'Connor), Mac Diarmata (MacDermott) of Mag Luirg, and Mac Donnchada (MacDonagh) of Tír Ailello.