The leprechauns are a species that appears in Post-SCrash Session 3: Spectators of the Host, as the primary inhabitants of the Third Creation. Because of this late-coming appearance, and likely because they do not reside in the primary Marrissaverse, they are not mentioned in Enchiridion Marrissa.
They are described as "green guys", sharing their skin color with the cherub trollz and Irkens. Otherwise, biologically/societally they're not that much different from the humans native to the Marrissaverse.
The leprechauns' mythology revolves around the fact that for tens of thousands of "sweeps", they've been waiting for a moment when "our planet's shell would spawn life all anew, showing us the Sylph of Space and the Knight of Time", referring to Cantaloupe and California, the last inhabitants of the Marrissaverse's Earth, appearing once it had been swallowed by the Red Miles.
Their architecture is generally a clash of various human architectural styles, showing the elements of the Sydney Opera House ("the seventy-sixth Cornucopia"), Notre Dame de Paris ("the Mona Lisa Church"), St. Basil's Cathedral ("that Russian spiral-ey building"), an igloo, the Empire State Building/"Tower", the Pyramid of Giza and the Westminster Clock Tower ("a pyramid which had a clock on each side").
They side themselves with Cantaloupe, rather than California, once the fight between the two breaks out.
- Post-SCrash Session 3: Spectators of the Host (First appearance)