3rd Leprechaun

The third one is on the Walking The Path level. He is located on a little peice of land right of one of the bridges. Use Gingerbread man to jump off the bridge onto the land and then talk to the Leprechaun.

All Bonuses

Enter code at scrapbook: Left, Up, A, X, Left, Up, A, X, Left, Up, A, X, Y, X, Y, X, Y, X.

Unlock All Levels

Left, Up, A, X, Left, Up, A, X, Left, Up, A, X, Up, Up, Up, Up, Up.

Refill Health Meters

Left, Up, A, X, Left, Up, A, X, Left, Up, A, X, Up, Right, Down, Left, Up.

1,000 Gold Coins

Left, Up, A, X, Left, Up, A, X, Left, Up, A, X, X, X, X, X, X.

Leprechaun Shops

There are 3 Leprechaun shops in Shrek 2. In each you can buy upgrades from Larry the Leprechaun.

1) Spooky Woods: go to the part where donkey kicks down two trees in a row. you should come to the place where the bats are. Before you pass the bats go to the right and you should meet Larry.

Upgrades:
Three Bears' Porrige: increases health maximum +3
Ogre Ring: increases attack strength +1
Pot of Gold: all coins collected worth double

2) Jail Break: go to the place where you roll the ball up the hill and free the pig. Then roll it down the hill and smash the gate. Go through and you will see Larry.

Upgrades:
Nine Lives: allows regeneration of health +8 per minute
Cupid's Arrow: increases attack strength +6
Lucky Rabbit's Foot: increases health maximum +9

3) i do not know where the last leprechaun is 
but these are the upgrades.

Upgrades:
Lucky Horseshoe: increases attack strength +4
Heart of Gold: increases health maximum +6
Green Clover: allows regeneration of health +8 per minute

2 Player

While your playing, pause and go to options. Go to character select and press A on the 2nd controller. Then chose a character.

Buy Upgrades

In some levels (such as Spooky woods) there is a leprechaun that will sell you a choice of three upgrades. He is not in every level, and sometimes he is hidden, so keep a sharp lookout!