Hi Morgain, I'm glqd to see you've made it to Brimloch Roon...
....You don't have to catch the Leprechaun....or kill him either. His treasure (the 'Stone of Clover') is in a chest, near Kerahs underwater shrine. You'll find it later in the game.
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...When you talk to Kerah, ( in Anephas' Shrine ) just try to rememeber to swim up and look for it...