Then you could make a trail of the gold leading up to either the Lucky coin box or the Fairy Cart. The leprechaun with collect the gold up to the box that you can prop open with a stick and some string. When he enters the box pull the string and you'll have caught him.
Or you could build the cart and have some small stuffed animal ready to help. (The Small Dog volunteered for me) Lead the coins up to the cart. When the leprechaun climbs inside have the small animal race around very fast in a circle until the leprechaun offers you a pot of gold to stop.
Another idea is that you could furnish a nice cottage made out of a box with fairy furniture . The leprechaun will be so grateful I'm sure that he would grant your wish.