Tadc or Tadg mac Briain was the son of Brian Boru

Brian Boru

Brian Bóruma mac Cennétig, , , was an Irish king who ended the domination of the High Kingship of Ireland by the Uí Néill. Building on the achievements of his father, Cennétig mac Lorcain, and especially his elder brother, Mathgamain, Brian first made himself King of Munster, then subjugated...

 and Echrad, daughter of Carlus mac Ailella of Uí Áeda Odba. Tadc had one son, Toirdelbach Ua Briain (Turlough O'Brien), with his wife Mór, daughter of Gilla Brigte Ua Maíl Muaid of Cenél Fiachach.

After Brian Boru's death at the battle of Clontarf

Battle of Clontarf

The Battle of Clontarf took place on 23 April 1014 between the forces of Brian Boru and the forces led by the King of Leinster, Máel Mórda mac Murchada: composed mainly of his own men, Viking mercenaries from Dublin and the Orkney Islands led by his cousin Sigtrygg, as well as the one rebellious...

 in 1014, Tadc was a serious contender to the kingship of Munster, rivalling his half brother Donnchad mac Briain

Donnchad mac Briain

Donnchadh mac Briain , formerly anglicised as Donough O'Brian, son of Brian Bóruma and Gormflaith ingen Murchada, was King of Munster.-Background:...

. Tadc was assassinated at the instigation of Donnchad in 1023.