When the Irish family heard the banshee's wail, they knew a death was near.

banshee's 是名词 banshee 的所有格形式，banshee 源自爱尔兰民间传说，指"报丧女妖"，被认为会在有人即将死亡时发出哀嚎声。's 表示所属关系，banshee's wail 即"女妖的哀嚎"。