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.
If you are bringing dessert to an adults-only party, consider making a cake with a piped image of a leprechaun enjoying his own green-tinted beer out of a large mug.
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.
Making a round cake into a leprechaun face is perhaps one of the simplest ways to design a cake around a leprechaun, especially for beginning decorators.