He first appears in 622, when the Annals of Tigernach report his presence at the battle of Cend Delgthen (probably in the east midlands of Ireland) as an ally of Conall Guthbinn of Clann Cholmáin. This is the only battle known where Domnall Brecc fought on the winning side.
Domnall suffered four defeats after he broke Dál Riata's alliance with the Cenél Conaill clan of the Uí Néill. In Ireland, Domnall and his ally Congal Cáech of the Dál nAraidi were defeated by Domnall mac Áedo of the Cenél Conaill, the High King of Ireland, at the Battle of Mag Rath (Moira, County Down) in 637. He also lost to the Picts in 635 and 638 and lastly to Eugein I of Alt Clut at Strathcarron in 642, where he was killed.
A stanza interpolated into the ancient Welsh poem Y Gododdin refers to these events:
I saw an array that came from Pentir,
And bore themselves splendidly around the conflagration.
I saw a second one, rapidly descending from their township,
Who had risen at the word of the grandson of Nwython.
I saw great sturdy men who came with the dawn,
And the head of Dyfnwal Frych, ravens gnawed it.
Domnall's son Domangart mac Domnaill was later to be king of Dál Riata and from him the later kings of the Cenél nGabráin were descended. A second son, Cathasach, died c. 650, and a grandson of Domnall, also called Cathasach, died c. 688.
- Eugein was the grandson of Nwython (Neithon).
- Anderson, Alan Orr, Early Sources of Scottish History A.D 500–1286, volume 1. Reprinted with corrections. Paul Watkins, Stamford, 1990. ISBN 1-871615-03-8
- CELT: Corpus of Electronic Texts at University College Cork includes the Annals of Ulster, Tigernach, the Four Masters and Innisfallen, the Chronicon Scotorum, the Lebor Bretnach (which includes the Duan Albanach), Genealogies, and various Saints' Lives. Most are translated into English and other translations are in progress.
| King of Dál Riata
Ferchar mac Connaid