I checked the 5E Monster Manual this morn', and to my surprise, no Leprechauns! You do see Pixies, to be sure, but no such luck for the true Leprechauns. Ah, what a loss for story potential! Larian, you pay no mind to the manual, go right on and use bonnie Leprechauns if it pleases you to do so.

(shamrock emoji goes here)