Sinead is the aunt of Siobhan Smythe.
When Siobhan started having visions of a banshee, she went to talk to Sinead at her magic shop "Luck of the Irish". Her aunt told her that one of their ancestors stole something from a banshee years ago. As a result, all of the women in the Smythe family are affected by a curse. Whenever someone wrongs them, it stirs up the spirit of a banshee. Sinead told Siobhan that her scream will grow more powerful until she can send it anywhere in the world. However, unless she kills the person who wronged her, it will destroy her soul. When Siobhan asked if this had ever happened to her, Sinead replied by implying that this was the reason that her husband William had died.