Tra la, troo lo, ye'll never catch me!
Sorry, couldn't resist..
He will not reappear. Once you've slain him, he's gone... until you ressurect him, that is. As long as you don't complete the resurrection ritual, even accidentally, you'll be fine.
Oh, you probably want to know what the ritual is now, don't you? Well, let me document it as officially as can be done...
To resurrect the leprechaun, follow these steps:
1. Undress completely (although you may wear a fancy hat, if desired)
2. Run around your house or apartment three times. This must be done during broad daylight.
3. Turn on your computer. Take the hat off if you have it on.
4. Run W&W.
5. Go to the Inn.
6. Press the "reset adventures button, wherever it is.
The Leprechaun is now resurrected, as is everything else in the game. Resetting adventures starts you over at the beginning with everything else you had, except certain quest items and guild memberships.
Oh, and steps one and two are optional, but can provide immense entertainment for others in the house.