Diarmait mac Máel na mBó

His father, Donnchad mac Diarmata, became known more commonly by the epithet Máel na mBó ("Baldy of the Cattle"), hence Diarmait's patronym.

The last of Diarmait's ancestors to have been counted as king of all Leinster, Crimthann mac Énnai, died in the late 5th century; but Diarmait's more immediate forebears, most recently his great-grandfather Domnall mac Cellaig (died 974), had been counted among the kings of the Uí Cheinnselaig.

Diarmait's mother was Aife, daughter of Gilla Pátraic mac Donnchada, king of Osraige.

He had at least one sibling, a brother named Domnall whose son Donnchad mac Domnaill Remair later became king of Leinster.

[3] The Uí Cheinnselaig had been prominent in earlier times, but their power had been broken at the battle of Áth Senaig in 738.

[7] The surviving sons of King Harold Godwinson of England escaped to Leinster after the Battle of Hastings in 1066 where they were hosted by Diarmait.