A slight alteration of the D&D flavour, though I admit it still doesn't really reflect the original folklore. I feel like I should bump her up to CR 17, and/or give her foresight as a SLA (to predict the deaths of others).

Hit Dice: 12d6+12 (54 hp)
Initiative: +3
Speed: 30 ft. (6 squares), fly 60 ft. (perfect)
Armor Class: 19 (+3 Dex, +6 deflection), touch 19, flat-footed 16
Base Attack/Grapple: +6/+6
Attack: Claw +10 melee (1d6)
Full Attack: 2 claws +10 melee (1d6)
Space/Reach: 5 ft./5 ft.
Special Attacks: Keen, spell-like abilities
Special Qualities: Damage reduction 10/cold iron, low-light vision, spell resistance 22, unearthly grace
Saves: Fort +11, Ref +17, Will +17
Abilities: Str 10, Dex 16, Con 12, Int 16, Wis 16, Cha 22
Skills: Hide +18, Intimidate +21, Knowledge (history, nature) +18, Listen +18, Move Silently +18, Perform (sing) +18, Spot +18, Survival +18 (+20 aboveground)
Feats: Ability Focus (keen), Improved Initiative, Improved Natural Attack (claws), Weapon Finesse, Weapon Focus (claws)
Environment: Temperate forests
Organization: Solitary
Challenge Rating: 12
Treasure: Standard
Alignment: Always chaotic (any)
Advancement: 13-18 HD (Medium)
Level Adjustment: +2

The woman is clad and in white and sobs to herself as she washes blood-stained garments in the river water. Your party paladin moves as though to comfort her, but your ranger holds him back - "No! Don't approach She is bean-sidhe."

Banshees are fey that are pitied and feared as harbingers of death. Their appearance often foretells the death of those who see them, though they can sometimes be persuaded to use their powers of divination for less-ominous ends.

Banshees speak Common, Elven, Gnome, and Sylvan.

Combat Abilities
A banshee starts combat by uttering her terrible wail, then claws at opponents with her dreadful talons.

Keen (Su)
As a full-round action a banshee may release a bloodcurdling scream of grief and rage. All creatures within 60 feet of the banshee must make a Fortitude save (DC 24) or die instantly upon hearing the keen. Once she uses this power, a banshee must wait 1d4 rounds before using it again. This is a sonic necromantic fear effect. The save DC is Charisma-based.

Spell-Like Abilities
1/day-augury, commune, divination, legend lore. Caster level 12th.

Unearthly Grace (Su)
A banshee adds her Charisma modifier as a bonus on all her saving throws and as a deflection bonus to her Armour Class. (The statistics block already reflects these bonuses).