For some time now, some of my on-line friends have advised me to provide a version of CassidySlangScam without the invective aimed at Cassidy and his supporters. In response to that advice, I am working on providing a glossary of the terms in Cassidy’s ludicrous book How The Irish Invented Slang with a short, simple and business-like explanation of why Cassidy’s version is wrong.
The late Daniel Cassidy, in his etymological hoax, claimed that English terms like puck for a spirit (as in Shakespeare’s A Midsummer Night’s Dream) are derived from the Irish word púca, a name for a kind of spirit well-known in Irish folklore. While there is no doubt about the word puck and the word púca being related (and discussion of this goes back a long way before Cassidy) there is little room for doubt that the ultimate origin of these terms is the Norse puki, meaning an imp. Remember that there are no native Irish words beginning with the letter p about from pus, which is a corruption of an earlier word bus.
cassidyslangscam 
Thorough research by Deasún Breatnach and others supports your conclusion that the Irish word púka comes from English (cf. Modern English Puck) rather than vice-versa:
Breatnach, Deasún. 1993. “The Púca: A Multi-Functional Irish Supernatural Entity.” Folklore. Vol. 104. No. 1/2. Pp. 105-110.
The first page of the article may be read here free of charge:
https://www.jstor.org/stable/1260800?seq=1
Hi David, Thanks for that! Somewhere in the house I have a copy of a book called “Chugat an Púca” by the same man. It’s a very interesting book. On the form of the word alone, we are talking about a loan from another language, whether that be English or Old Norse. But of course, whenever there is a similarity, some people (the Cassidy fan-club included) will always claim that the route of transmission had to be from Irish into other languages and that people like us are bigots for not accepting that. Unfortunately for these people, there is evidence and it is quite clear. There are effectively (I have discussed the exception of pus elsewhere) no Irish words beginning with the letter p apart from words borrowed from other languages. That’s a fact. I know it, you know it, Breatnach knows it, anyone with any knowledge of the language or of linguistics in general knows it. Not that that will stop them arguing to the contrary!
Hope all is well there! 🙂