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.
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.
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.
Guys who don't want to wear full body leprechaun mascot costumes while they bar hop on St. Patty's Day might consider wearing a traditional shirt and jeans, while rocking the holiday with shamrock shades, a green wig or a shamrock top hat.