A banshee ( BAN-shee; Modern Irish bean sí, from Old Irish: ban síde, “woman of the fairy mound” or “fairy woman”) is a female spirit in Irish mythology who heralds the death of a family member, usually by shrieking or keening. Her name is connected to the mythologically important tumuli or “mounds” that dot the Irish countryside, which are known as síde (singular síd) in Old Irish.
 
The Banshee, bean-sidhe (woman of the fairy may be an ancestral spirit appointed to Banshee. Fantasy style portrait of a howling womanforewarn members of certain ancient Irish families of their time of death. According to tradition, the banshee can only cry for five major Irish families: the O’Neills, the O’Briens, the O’Connors, the O’Gradys and the Kavanaghs. Intermarriage has since extended this select list.
Whatever her origins, the banshee chiefly appears in one of three guises: a young woman, a stately matron or a raddled old hag. These represent the triple aspects of the Celtic goddess of war and death, namely Badhbh, Macha and Mor-Rioghain.) She usually wears either a grey, hooded cloak or the winding sheet or grave robe of the unshriven dead. She may also appear as a washer-woman, and is seen apparently washing the blood stained clothes of those who are about to die. In this guise she is known as the bean-nighe (washing woman).

Although not always seen, her mourning call is heard, usually at night when someone is about to die.
 
Folklore - Banshees. Date: 1862
Bunworth Banshee, “Fairy Legends and Traditions of the South of Ireland”, by Thomas Crofton Croker, 1825