In 1071, the year before his grandfather's death, Domnall and an Uí Chennselaig kinsman, Donnchad mac Domnaill Remair, battled for control of Leinster.

He may have collaborated with Gofraid to wrench the kingdom from the grip of the Uí Briain, or he may have been installed in the kingship by Toirdelbach himself, and ruled under the latter's overlordship.

[13] Diarmait's deep-rooted authority in Norse-Gaelic Dublin lasted for two decades, and was a remarkable achievement that no other Irish king had ever accomplished.

[17] Specifically, the seventeenth-century Annals of the Four Masters,[18] and the eleventh- to fourteenth-century Annals of Inisfallen reveal that Domnall battled against the forces of his own first cousin once removed, Donnchad mac Domnaill Remair, before Diarmait's ally, Toirdelbach Ua Briain, King of Munster, was able to intervene and restore order in the Kingdom of Leinster.

[20] In consequence of the void left by his demise, Diarmait's erstwhile ally Toirdelbach seized the initiative, and moved to enforce his own claim to the high-kingship of Ireland.

[21] He immediately imposed his overlordship on Leinster[22]—a task almost certainly expedited by the aforesaid infighting amongst the Uí Chennselaig[20]—and took control of Dublin.

Toirdelbach's subsequent capture of Donnchad in Dublin suggests that the latter was not only the leading Uí Chennselaig dynast,[30] but was also in the process of using the town as the capital of Leinster.

[33] Although this record may be mere Uí Briain propaganda, it could instead reveal that the Dubliners preferred a distant overlord from Munster rather than one from neighbouring Leinster.

[40] On one hand, it is possible that Gofraid was involved in lending assistance to Anglo-Danish resistance against the Norman regime in the recently conquered Kingdom of England.