Máel Sechnaill mac Domnaill, also called Máel Sechnaill Mór, Máel Sechnaill II, anglicized Malachy II, was King of Mide and High King of Ireland. His great victory at the Battle of Tara against Olaf Cuaran in 980 resulted in Gaelic control of the Kingdom of Dublin.
Máel Sechnaill belonged to the Clann Cholmáin branch of the Uí Néill dynasty. He was the grandson of Donnchad Donn, great-grandson of Flann Sinna and great-great-grandson of the first Máel Sechnaill, Máel Sechnaill mac Máele Ruanaid. The Kings of Tara or High Kings of Ireland had for centuries alternated between the various Uí Néill branches. By Máel Sechnaill's time this alternating succession passed between Clann Cholmáin in the south and the Cenél nEógain in the north, so that he succeeded Domnall ua Néill in 980. This system had survived previous challenges by outsiders including the kings of Ulster, Munster and Leinster, and the Viking invasions.
The system of alternating succession between the various Uí Néill branches was ended by Brian Boru's so-called overthrow of Máel Sechnaill in 1002. In fact this was a bloodless shift resulting from the failure of the Northern Uí Néill, his kinsmen, to support Máel Sechnaill against the aspirations of the extremely militarized overlord of Munster. Brian would have little more success with them himself.
Because of the death of Brian Boru, his son, grandson and many other Munster nobles at Clontarf in 1014, Máel Sechnaill succeeded in regaining the titular High Kingship, with the aid of his northern kinsman Flaithbertach Ua Néill, but the High Kingship, albeit with opposition, did not reappear until Diarmait mac Maíl na mBó of Leinster rose to power. Clann Cholmáin provided no further High Kings, but the northern Uí Néill of the Cenél nEógain provided two: Domnall Ua Lochlainn and Muirchertach Mac Lochlainn.