Leprechaun gold is a fake type of gold created by Leprechauns, who have the ability to produce gold coins that look and feel authentic as regular coins which look like Galleons, certainly in the wizarding community. It has the quality of disappearing after a few hours; not many wizards and witches seem to be aware of this fact, as Ron Weasley (who is usually more aware of the magical world than his two best friends: Harry Potter and Hermione Granger) was unaware until Hagrid brought it up during a Care of Magical Creatures lesson. Although the coins seem identical to ordinary ones, goblin experts can differentiate them from ordinary coins, as Harry Potter, Hermione Granger, and Ron Weasley overheard when they visited Gringotts Wizarding Bank in search of one of Lord Voldemort's Horcruxes in the Lestrange vault.
Ron Weasley paid back Harry Potter in Leprechaun gold for the Omnioculars in the Quidditch World Cup, not knowing that Leprechaun gold would disappear until Hagrid told him later in Care of Magical Creatures class. Ron felt bad about this, but Harry told him it was no problem.
Behind the scenes
The concept of leprechaun gold vanishing after a time may draw from the concept in Welsh folklore that money or other gifts given by fairies will vanish or turn to worthless objects if you tell anyone where you got them from.