According to legend, a banshee wails around a house if someone in the house is about to die.

Traditionally, when a citizen of an Irish village died, a woman would sing a lament (in Irish: caoineadh, ) at their funeral. These women singers are sometimes referred to as "keeners". Legend has it that, for five great Gaelic families: the O'Gradys, the O'Neills, the O'Briens, the O'Connors, and the Kavanaghs, the lament would be sung by a fairy woman; having foresight, she would appear before the death and keen. When several banshees appeared at once, it indicated the death of someone great or holy. The tales sometimes recounted that the woman, though called a fairy, was a ghost, often of a specific murdered woman, or a woman who died in childbirth.

Banshees are frequently described as dressed in white or grey, and often having long, fair hair which they brush with a silver comb, a detail scholar Patricia Lysaght attributes to confusion with local mermaid myths. This comb detail is also related to the centuries-old traditional romantic Irish story that, if you ever see a comb lying on the ground in Ireland, you must never pick it up, or the banshees (or mermaids - stories vary), having placed it there to lure unsuspecting humans, will spirit such gullible humans away. Other stories portray banshees as dressed in green, red or black with a grey cloak.

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. In 1437, King James I of Scotland was approached by an Irish seeress or banshee who foretold his murder at the instigation of the Earl of Atholl. This is an example of the banshee in human form. There are records of several human banshees or prophetesses attending the great houses of Ireland and the courts of local Irish kings. In some parts of Leinster, she is referred to as the bean chaointe (keening woman) whose wail can be so piercing that it shatters glass. In Kerry, the keen is experienced as a "low, pleasant singing"; in Tyrone as "the sound of two boards being struck together"; and on Rathlin Island as "a thin, screeching sound somewhere between the wail of a woman and the moan of an owl".

The banshee may also appear in a variety of other forms, such as that of a hooded crow, stoat, hare and weasel - animals associated in Ireland with witchcraft.