As far as I know the Fountain of Isis is only a Malakai quest.
The leprechaun is a quest your thief can get. Actually what you need to do to solve that quest is find his treasure which is on the far side of the pyramid. You don't seem able to get any clues from the leprechaun himself and just exploring will eventually bring you to the treasure. Not a great quest in my opinion.
Wyv
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