This book is what we call codology. There is no doubt that the Irish did make a contribution to the slang of America. Policemen traditionally called their truncheons shills in America (from shillelagh, an anglicised form of sail éille), and the Irish origins of snazzy (from snas which means polish; snasta means polished or snazzy) and the possible Irish origins of dude were in the public domain a long time before Cassidy came along. But their contribution was far more limited than Cassidy suggests. While his explanations seem convincing at first glance, the supposed Irish phrases or words he gives are often nonsense, the result of plundering dictionaries for slight similarities of sound rather than any deep knowledge of the language. Teas does mean heat, and can occasionally suggest sex, as in the 17th century priest's poem - tig ón teas an toil (from heat comes desire). However, heat and semen are quite different things, and ioma can't be used like this. You can't say obair ioma for too much work, or airgead ioma for too much money. Is lom é might just mean at a pinch "it's bleak" (though lom has lots of different meanings - it could just as easily mean "he's naked"), but is it likely that this phrase would change its function completely and become a noun meaning slum? Hardly. It originally meant a cheap room, so surely some connection with slumber is a better bet? And sometimes the book sails very close to outright dishonesty. For example, he suggests that loingseoir, which he claims means "maritime worker" is the origin of longshoreman. Now, not only does longshoreman have a perfectly respectable and convincing origin (that the longshoremen were the along-shore men, the people working on the dock), but no Irish dictionary would define loingseoir as maritime worker. A loingseoir is a sailor. The difference is subtle but all-important. Longshoremen aren't sailors, but they could clearly be called "maritime workers". As for bocaí rua for buckaroo (which EVERYONE except Cassidy accepts comes from vaquero, a cowboy in Spanish), why rua? What the Hell does being red-haired have to do with being a cowboy?? (Because that's what rua means, not gory or bloody). Unfortunately, this welter of fake etymologies will prevent scholars from looking for possible Irish derivations for words for fear of being lumped together with Cassidy. And that's a pity, because there are genuine Irish derivations which have been ignored by the world of scholarship, like the word conk, which the OED suggests is linked to conch. The Irish cainc meaning a big nose is a more likely contender.
The weirdest thing about all this is not that Cassidy produced such a tissue of bizarre nonsense, but that so few people have had the sense to realise what it is.