Domnall mac Brian

FMG states the following:

"Brian and his [first/second] wife had one child: Domnall (? - 1011).
The Annals of Inisfallen record the death in 1011 of "Domnall son of Brian.[1003] The Annals of the Four Masters record the death in 1010 of Domhnall son of Brian son of Ceinneidight, son of the king of Ireland.[1004] The name of Domnall's wife is not known. Domnall and his wife had one child: Toirdelbach ([1008/1009 - CennCoradh 14 Jul 1086)...."
Annals of Inisfallen, 1011.3, p. 181 [https://celt.ucc.ie/published/T100004/index.html]
Annals of the Four Masters, 1010.12, p. 765 [https://celt.ucc.ie/published/T100005B/index.html]
The reason for differing death years is unknown. While the Julian calendar was in use in 11th-century Ireland, the difference in years cannot be explained by the 1582 origin of the Gregorian calendar. These Annals were penned before 1582. Moreover, the Gregorian calendar was not embraced by all nations and churches for over 100 years.