Domnall was a son of Murchad mac Diarmata, King of Dublin and the Isles,^[6] who was himself a son of Diarmait mac Máel na mBó, King of Leinster.^[7] Domnall was, therefore, a member of the Uí Chennselaig;^[8] as well as the first of the Meic Murchada, a branch of the Uí Chennselaig^[9] named after his father.^[10] Domnall had two brothers: Donnchad, a later King of Leinster, and Énna.^[11]

In 1052, Domnall's aforesaid grandfather conquered the Kingdom of Dublin from Echmarcach mac Ragnaill, King of Dublin and the Isles, and soon after appointed Murchad as King of Dublin. About a decade later, Murchad appears to have driven Echmarcach from Mann,^[12] after which he gained the kingship of the Isles.^[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.^[14] Unfortunately for the Uí Chennselaig, two of Diarmait's sons—Murchad and Glún Iairn—unexpectedly predeceased their father in 1070,^[15] and Diarmait himself fell in battle two years later.^[16]

Even before Diarmait's demise, the Uí Chennselaig began to fight amongst themselves in a struggle that was almost certainly an after-effect of Diarmait's sons' untimely deaths.^[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.^[19]

Up until about the time of his death, Diarmait had been the most powerful king in southern Ireland.^[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.^[23] Whilst the imposition of authority upon rival provincial kingdoms was a fundamental part in gaining the high-kingship, Toirdelbach's decision to march-on Dublin reveals that the acquisition of this coastal kingdom had also become an essential part of the process.^[24]

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.^[31] Although the list of Leinster kings in the twelfth-century Book of Leinster declares that Domnall had succeeded his grandfather as King of Leinster, it is apparent that Donnchad was indeed the more powerful claimant. In fact, the king-list of Uí Chennselaig in the same source makes no notice of Domnall, and states that it was Donnchad who succeeded Diarmait as King of Uí Chennselaig. Domnall, therefore, may not have reigned in Leinster, and could well have been King of Leinster in name only.^[32]

If the Annals of Inisfallen is to be believed, Toirdelbach acquired possession of Dublin when the Dubliners themselves offered him its kingship.^[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.^[31] Within the year, the kingship was held by Gofraid mac Amlaíb meic Ragnaill.^[34] The latter appears to have been a kinsman of Echmarcach,^[35] and may well have been handed the kinship by Toirdelbach,^[36] perhaps on account of the considerable distance between the kingdoms.^[37]

In 1075, Toirdelbach drove Gofraid from the kingship and Ireland itself.^[39] There is uncertainty concerning the circumstances of Gofraid's expulsion, and of Domnall's accession.^[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. If correct, Gofraid would appear to have been at odds with Toirdelbach, a monarch who appears to have cultivated close links with the Norman regime.^[41] Domnall, therefore, may have had Toirdelbach's consent to rule in Dublin as Gofraid's replacement.^[42] In fact, Toirdelbach's placement of Domnall in Dublin, and his allowance of the latter's aforesaid cousin in Leinster, may have been a way in which the Uí Briain exploited the fractured Uí Chennselaig. Certainly, Domnall's cooperation would have been a valuable asset to Toirdelbach, considering the prominence of his father amongst the Dubliners, and the likelihood that Domnall himself may have lived most of his life there.^{[43][note 3]} On the other hand, it is possible that Gofraid was driven from the kingship because he had aligned himself with the Leinstermen against the Uí Briain. If such a sequence of events is correct it could mean that, even though Gofraid was unable continue on with the revolt, it was his Uí Chennselaig confederates who succeeded in securing Dublin from the Uí Briain.^[40]

Whatever the circumstances of Domnall's accession, the Uí Chennselaig regime in Dublin was short-lived. The Annals of Inisfallen, the Annals of the Four Masters, and the fifteenth- to sixteenth-century Annals of Ulster, all reveal that, within the year, Domnall died after a brief illness, with the latter two sources specifying that he succumbed after three nights of sickness.^[45] The Annals of Inisfallen and the Annals of Ulster accord him the title King of Dublin, and make no mention of any connection with the Leinster kingship.^[46] Upon Domnall's demise, Toirdelbach had his own son, Muirchertach, appointed King of Dublin.^[47] In so doing, Toirdelbach reinforced his authority in Dublin,^[48] and followed a precedent started by Domnall's grandfather, in which a claimant to the high-kingship of Ireland installed his own heir to the kingship of Dublin.^[49]

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