You’ve all heard the stories…
The slow crunching footsteps beneath me are the only noise on the dirt road. It winds, twisting and turning through the fields to my home. It’s dark, and the walk is long and lonely. I have about a mile to go before I reach the comfort of the cottage. The thought of warm stew and potatoes makes my mouth water, and my arms shiver, as I clutch my jacket tighter around me in the cold.
I stop abruptly, the blood in my viens turning to ice at what I thought I heard. My heart beats quickly in my ears, the only sound beside the rustling of the leaves on the branches above me. I must have imagined it, I think to myself. Surely it isn’t time yet…
There it is again! The wailing cry of the bean sí pierces the night, louder this time. She’s getting closer, and I can’t stay still for long. I break out into a sprint, running as fast as I can up the road. She keeps wailing, her keening filling the air around me. It’s cold, and my breath comes out of my mouth in warm puffs of white air.
The hair on my arms stands up on edge as I crest the hill, my house standing in the distance. The lights are on inside and smoke rises from the chimney. There are people gathered by the door, a crowd that shouldn’t be there.
It was the bean sí, and someone is not long for this world.
– - Megan Black
By W.H. Brooke - Public Domain, https://commons.wikimedia.org/w/index.php?curid=5700663
The literal translation of Bean Sí (anglicized as Banshee) is “woman of the mounds”. In Irish folklore, she is a female spirit that heralds the death of a family member by wailing or keening. According to Jones’ Celtic Encyclopedia , she is usually dressed in a gray cloak and green dress with red eyes from all her weeping. Jones’ Celtic Encyclopedia also says that the Bean Sí is usually associated with one family and that when you hear her keening, someone in that family was going to die.
W.B. Yeats describes the Banshee as follows:
“The banshee (from ban [bean], a woman, and shee [sidhe], a fairy) is an attendant fairy that follows the old families, and none but them, and wails before a death. Many have seen her as she goes wailing and clapping her hands. The keen [caoine], the funeral cry of the peasantry, is said to be an imitation of her cry. When more than one banshee is present, and they wail and sing in chorus, it is for the death of some holy or great one. An omen that sometimes accompanies the banshee is the coach-a-bower (cóiste-bodhar)–an immense black coach, mounted by a coffin, and drawn by headless horses driven by a Dullahan. It will go rumbling to your door, and if you open it, according to Croker, a basin of blood will be thrown in your face.” 
By R. Prowse - Wicklow Heritage, Public Domain, https://commons.wikimedia.org/w/index.php?curid=110428480
There was speculation that the Banshee may have been an ancestor of the family at one point in time, though I don’t know for sure. Some sources say that the only families that have a Banshee are those that begin with O’ and Mac, though other sources say that isn’t true. This shows that the Banshee is not one being but possibly many, with each family having its own. However, not every family will have a Bean Sí and you most likely will not find a Bean Sí outside of Ireland.
And here are some quotes from Duchas, the collection of Irish folklore.
About twenty years ago, my father heard a banshee. A women named Mary O Donnell was sick. The banshee was walking on the tops of the bushes. It was in the Sumer-time and my father was milking the cows outside in the fields. When the cows heard the banshee, they began to run through the fields and they would not let him milk them. So he had to bring them into the byre to milk them. In an hour after, the banshee went over to the house and tapped at the window and the girl died. Then the banshee disappeared. 
The above story shows that just because you hear a bean sí doesn’t mean it’s someone in your family that is going to die. As some of the sources say, the bean sí is only attached to specific family lines in Ireland. Some say those are the oldest familial lines. Others, like the story below, give those specific names. Yet still others say that many families have a bean sí, but not all.
The old people around this district firmly believed in the Banshee. I often heard my mother say that there were certain families that were always “cried”. These families were the McGowans, the Gallyaghers, the Keanys, the Meehans and the O’Briens I heard my father say that he and an uncle of his heard the banshee crying one night just before an old Keany woman died. A McGowan lad lived beside us. He had a sister married down near Derrygonnelly, Co Fernanagh. Before this sister died the story goes that they heard the banshee. I also heard other people saying that they heard the banshee and it always foretold the death of some person.
I had a strange personal experience in this connection. On the night of the 14th of March 1919 (the time of the big flu epidemic) my sister and I went out at 10 o’clock to get some turf to “rake the fire”. Suddenly we heard a most unearthly cry. It started about a mile away from us and ran along the ground for about half a mile away from us and ran along the ground for about half a mile. Then it began to ascend and went up, up, up getting fainter as it went until it dies away in the sky. We never heard anything so weird and concluded that it must be the banshee. We went home and told our people that we heard a banshee. They laughed at us. It happened that our next door neighbor, Mrs. D - whose maiden name was Gallaghen took the flu that night and was dead that day week. 
Some people have brushed off the story of the bean sí only to realize later that it was a mistake. Either that, it was pure coincidence!
"Long ago I heard the people saying that the Banshee crys like a child. The people do not like to hear the Banshee crying because it is a very frightful cry. I often heard the people saying that there is a race of people that the Banshee follows when they are going to die.
Once upon a time I heard the people saying there was an old man lived at Greenhill and one night he was going to St. Johnston and he heard a great noise and at first he thought that it was a child crying. He told the people and they went to hear the cry and there was a man there that heard the Banshee before and he said that it was the Banshee. A few days after that the old man got sick and he soon died and he was one of the people that the Banshee followed. 
Whether you believe in the bean sí or not, she has played a large role in the folklore in Ireland. There are stories of the bean sí all over the country, and some of those stories have followed the diaspora over to the United States. You can even see pop culture depictions of the Banshee in shows like The Vampire Diaries.
I do want to point out that this spirit, this woman of the mounds, is not a spirit to be summoned or messed with in any way. She is not a guide. She is not a member of the Sídhe as we know Them, and she is not a psychopomp to the Otherworld. Despite what one popular author has written, you should not summon a bean sí to act as a death doula or tour guide of the Otherworld before you pass. That isn’t her role and she most likely can’t be summoned anyway.