On his father's side.
Before he known as Bronach, he wasn't Bjorn. A promisingly young son of a line of Viking chiefs. His father, Asger Stone Heart, was a battle scared warrior. A man of feeble words, but had a fist of iron. His wife and shield maiden, Helga Blood Print, was a fierce one legged woman. Even after losing a leg, she patched it with a peg and continued to raid. Bjorn would compete for respect among his bloodthirsty older brother, Hakon Blood Fist.
Bjorn never liked raiding, but they had no choice. Food was scarce, territory was low. The winters were so harsh.
One night during a banquet, his lord's right hand man was raping a slave girl. Bjorn had enough, and killed him. Saving the girl. But this all came at a grave price. The murder brought grave shame to his family. The lord requested a blood tribute for his loss. Bjorn left. Escaping to the sea, never to return. He changed his name said little about his past. Bronach.... Sorrow.
His brother was sent back to kill him and restore honor. But years passed by, until Hakon attacked Bronach's village. Killed him and his wife and was about to kill his nephew too. But Cellach stabbed and killed him, saving Brendan.
The Secret of Kells (C) Cartoon Saloon & Tomm Moore.