If you want to be a leprechaun out of the traditional tales, Buy Costumes also has a leprechaun costume with a scary mask included, guaranteed to induce nightmares.
Alternatively, use fondant to create a rainbow that goes up and over a tiered cake, with a leprechaun made of sculpting fondant or modeling chocolate sitting on one of the middle tiers below.
In Ireland, on a summer's morn, I saw a tiny leprechaun.
The leprechaun is designed to make the wearer appear shorter than he or she really is, so you will see through the mesh opening in a shamrock on the hat if you wear this costume.
Just like the legendary leprechaun, they're small!