To unlock the Crossy Road secret character LEPRECHAUN, you have to be playing as a UK character (any character from the update, excluding LLAMA and #THEDRESS.) While playing, you will see clovers on the ground. Occasionally there will be a four leaf clover. If you pick up this clover, you will unlock the LEPRECHAUN.
Click on the category "Secret Crossy Road" to learn how to unlock the other characters!