A banshee is a spirit from Celtic mythology. Traditionally the banshee takes the form of a woman and is known to wail a warning of death to those who are living. There are different beliefs as to what a banshee is, whether or not it is an ancestral spirit or a faerie from the other world. Some believe that only a select few families can inspire a banshee to bring a warning of death, while others feel that it’s just a matter of having Irish blood in you, no matter the family.
Not all banshees mourn. There are both joyous and mournful banshees. In Wentz’s Fairy Faith in Celtic Countries, there is a legend that after the Macleod’s had borne an heir, a beautiful woman appeared at the castle, went to the newborn’s cradle and chanted a series of verses over the baby. The verses fortold the future manhood of the young child and acted as a protective charm over the young one (Sleeping Beauty anyone?). After having done this she put the baby back into it’s cradle and disappeared across the moorlands.
This website goes over the banshee in greater depth than what I’ve provided. http://www.myremoteradio.com/blog/the-banshee
And in even greater detail http://www.monstropedia.org/index.php?title=Banshee
A banshee makes an appearance in the book Hunter’s Moon by O.R. Melling