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.
In Ireland, on a summer's morn, I saw a tiny leprechaun.
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.
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!