According to Daniel Cassidy, author of the atrocious How The Irish Invented Slang, the English slang term spiel (meaning a rigmarole or speech) doesn’t come from a German or Yiddish term meaning play. He offers no evidence to disprove this claim. Instead, he asserts that the Irish word speal is the origin of this term. Speal means a scythe, an agricultural implement used for cutting barley or oats or hay. It is pronounced to rhyme with Hal, not with heel, so it sounds quite different.
It does not mean to use cutting words or anything of the type in Irish. Neither Ó Dónaill’s dictionary nor Dinneen’s dictionary give this as a meaning of speal or any related word. Dwelly’s Scots Gaelic dictionary does give this as a subsidiary meaning. Of course, a spiel is not usually insulting or cutting – by its very nature, a spiel has to be persuasive. And again, even if Cassidy is right and the German or Yiddish explanation is wrong (which is very unlikely) this would have nothing whatever to do with Irish influence on American slang, because speal isn’t used in this way in Irish.
In short, it’s just more old crap from Cassidy.