Take the Leprechauns hat (which is his most valuble item) and he'll beg for it back. Don't give it back to him and say give me your gold and I will give you your hat. He will give you his gold and you will have to give him his hat!
the rumor that they have only one pot is a myth, the leprechauns want you to think they only have one pot so when you catch them you only ask for a single pot of gold in exchange for their freedom...