I’m afraid the book has more entertainment value than anything else. The guy who wrote it, while very enthusiastic, didn’t actually speak Irish, and had no background in linguistics. The book is mainly based on speculation, and most of his conclusions are considered to be pretty “out there” by Irish scholars.
. . . cuts through two hundred years of Anglo-American academic “baloney” and reveals the massive, hidden influence of the Irish language on American vernacular and slang. Irish-derived words and phrases – like snazzy, swell, dude, scam, slum, say “uncle”, sucker, knickknack, twerp, nincompoop, moolah, racketeer, rookie, ballyhoo, dork, freak, hoodoo, Dead Rabbit, and jazz – are scattered across the American language . . .
Oidh bheidh.
My only question is whether this is innocent armchair etymology as expression of ethnic pride, or something that veers into Barry Fell/Welsh-speaking Indians territory.
The sad thing is, while there IS a good case for some English slang words being derived from Irish (“snazzy” from “snasta” springs immediately to mind, and “smashing” from “is maith sin”), he goes too far in his speculations.
Well yes, the Scottish Gaels also claim “'s math sin” and “snasail”, for example. I’m sure in several cases, where the forms are similar and used with equal frequency, it wouldn’t be possible (or productive) to try to distinguish which variety “made it” into English.
I would say that the strongest argument for Irish being the source of some of these is the sheer number of Irish speakers who immigrated (or were transported) to the U.S., England and Australia vs. the number of Scottish Gaelic speakers, especially from the mid-19th century forward. When and where a particular term first cropped up would probably be key.