Pot Of Gold

The Leprechauns lost their gold to humans who followed rainbows right to them.
Leprechauns have long memories and even longer lives, so they worked with what little they had left to make real estate deals and investments to build up their fortunes once more.
Late-night advertisements offered cash for gold, luring desperate humans to mail the shiny metal back to the Leprechauns.
They learned their lesson with the rainbow-attracting pots, and built massive secure vaults deep under Ireland to store their treasure.
To keep the curious humans at bay, they set out a few pots filled with poison-coated Iron Pyrite.