"Monies", or alternatively "moneys", is the term used by Irkens to refer to their form of currency, revealing that Irk has an economy and that they engage in trade negotiations with foreign planets. It's also suggested in "Hobo 13" that the Irken currency itself is called "monies."
Irken Monies have a picture of the current Almighty Tallest on them. This brings up the possibility that, during the reigns of Almighty Tallest Miyuki and Spork, the Monies printed bore their respective likenesses.
Also, they only appear to be in coin form, as its unknown what printed monies look like, although in "The Frycook What Came from All That Space" some alien costumers could be seen paying Sizz-Lorr during "The Great Foodening" with what one could assume are Irken monies, these monies seems to resemble dollars.
In "Mysterious Mysteries", Zim's side of the story stated that Dib demanded to him to give him his lunch "monies", and in "The Most Horrible X-mas Ever", Zim told a human in a Santa suit to "stop stealing all my monies!"
It is possible that Irkens rarely use physical monies, as in "Tak: The Hideous New Girl" it was shown that Zim used his PAK to pay for a snack in a vending machine while on planet Devastis, and in Issue 35 Zim used a credit card-like device called a paything to settle the bill he had for the prisoners he was paying for to be incarcerated in Moo-Ping 10.