Siobhan was the lone daughter of a blacksmith. When her father died, she took over the family business, which chased away several potential pursuers. However, she did not mind - she had no intention of getting married.
Her beauty and impressive strength and combat skills that scared away so many men in her village attracted the attention of Sancar, a vampire who was building a harem, and chose her as one of his women. Her strength, however, gave him a hard time - she was much stronger than the average newborn, and she showed no love for him like his other followers. Sancar managed to keep her distracted with blood, but before her newborn year ended, she killed him and subsequently three of his followers before setting off on her own. Knowing she could not go back home, she explored the world.
During one of these travels, she encountered some of the Volturi's guards and became fascinated with how certain vampires could develop special talents different from other vampires.
Liam was a warrior of an Irish army fighting during the Irish Rebellion and later the Cromwell's reconquest back in the mid-1600s.
During one of the battles, his troop was attacked by a nomadic vampire who was using the war as a disguise to feed without attracting notice. Liam was the last to be bitten, but because the vampire was already satiated, he did not finish him off and Liam was left barely alive. The bite, however, turned him into a vampire, and he had to learn to adjust to his new cravings and abilities on his own. However, his days of ignorance ended when he met Siobhan.
Liam was still a newborn when he first came across Siobhan. Siobhan took him under her wing and taught him everything that she'd learned about being a vampire, the advantages and the laws. Over time, the two bonded as a couple, and then they added Maggie to their coven: Siobhan convinced Liam to allow her involvement and they eventually learned to love the girl.
- Main article: Breaking Dawn
Siobhan and Liam, and Maggie are sought by Carlisle and Esme Cullen. With no clear explanation, they send the trio to Forks where their adopted children do the explaining. At Maggie's confirmation of their truthful story, the three agree to help the Cullens testify against the Volturi's accusations.
During the confrontation, they stand next to each other as Siobhan converses with Aro about Renesmee Cullen's human and vampire traits. After the situation ended peacefully, Liam and Siobhan return to Ireland with Maggie, but agree to find Alistair and tell him how everything played out.