Leprechaun hair is a material from the head of a Leprechaun, and it can be used, albeit uncommonly, to comprise a wand core. Noted to be only used in Irish wands, Garrick Ollivander himself refuses to use Leprechaun hair, as they are noted to make for extremely temperamental wands that are prone to releasing magic without the wizard's consent — the magic itself will often be of random effect, and can either be "lucky" or "unlucky" depending on the whim of the wand. Due to the possibility of accidentally hitting a Muggle with a nasty Stinging Hex being just as likely as somehow adding a new chimney to the nearest Muggle dwelling, it is not a favored core among makers.
Despite this, Leprechaun hair wands are noted to be fiercely loyal to their owners, though the nature of the core can clash with the wood, making for a difficult wand to deal with.
- Leprechaun on Harry Potter Wiki