Cassidy worked for the New York Times as a news assistant. He was a professional musician, starting as a reed player, and cutting an album as a singer and composer. He played Carnegie Hall, the Civic Auditorium, and The Tonight Show – performing with comedian George Carlin, Kenny Rankin, and Lilly Tomlin.
His work appeared in the The San Francisco Chronicle, the New York Observer and the Atlantic Monthly.
Cassidy died of pancreatic cancer at his home in San Francisco.
- 2007 American Book Award
- How the Irish Invented Slang: The Secret Language of the Crossroads. CounterPunch Books and AK Press. July 2007. ISBN 978-1-904859-60-4.
- "Civil Rights and Civil Wrongs"
- "Uncensored Voices,"
- "Dan Cassidy" was released by Little David Records (LD 1002) in 1972. Billboard said, "Dan Cassidy projects an immense strength and a rare understanding of the human predicament circa early 1970s on this his initial album effort. He's seen his share of the unpretty side of life and his lyrics reflect this with the utmost sincerity and compassion."
Cassidy's book is one of those eureka moments that leap beyond the ordinary to give us a new understanding of the subject at hand.
So utterly, completely stupid, only a total nincompoop like Cassidy could have come up with it.
… the problem with the book is that there is no scholarship or real evidence at all in it.
Tá Cassidy imithe ar shlí na fírinne anois, is é mo mhórdhóchas ná go n-imeoidh na bréaga seo leis. Ná ceannaigh an leabhar seo is ná tacaigh le seafóid mar seo. (Cassidy has passed away now, it is my fervent hope that these lies will pass away with him. Do not buy this book and do not support nonsense like this.)
Cassidy’s main thesis—that there are far more English words of Irish origin than are acknowledged in volumes such as the OED, and that this is due to the fact that much of the Irish influence is found among lower-status, colloquial slang expressions—is very convincing, but the etymologies he proposes for individual words would require a substantial amount of research before they could be taken as fact.
Cassidy paints himself as the maligned scholar, the unappreciated genius, the outsider. He may be all of those things, but he is them by choice: his work cannot withstand scholarly scrutiny so he simply cannot afford to join forces with any larger body of experts who do this sort of thing for a living. His book falls apart on first reading by anyone with some expertise in the field.
Etymologies from Cassidy's How the Irish Invented Slang are widely duplicated across the internet. However, many of Cassidy's definitions have been shown to be wishful thinking or completely made up. Cassidy was not able to speak Irish himself and was unfamiliar with the grammatical rules. In many cases, he apparently found words in Irish dictionaries that he thought had a similar pronunciation to English words or phrases with a vaguely connected meaning. He then claimed these English words to have an Irish origin even when the English word already had a well established etymology. However, in the majority of cases, he simply combined Irish words into phrases which he thought resembled English slang terms but without providing any evidence that these phrases ever existed in Irish Gaelic.
Although his theories appeal to the idea that the contributions of working class Irish immigrants to US English have been ignored by English-speaking lexicographers, they have been heavily criticized by academics. These include the American lexicographer Grant Barrett and Irish lexicographer Terence Dolan, Professor of Old and Middle English at University College Dublin.
- Doyle, Jim (15 October 2008). "Irish American Daniel Cassidy dead at 65". SFGate.com. San Francisco Chronicle, Hearst Communications Inc. Archived from the original on July 3, 2009. Retrieved 7 April 2016.
- "An Unqualified Failure". cassidyslangscam. Retrieved 2015-03-30.
- Cassidy, Daniel (July 29, 1998). "Churches of Fire in Ireland and the South". The San Francisco Chronicle.
- "Album Reviews". Billboard. Los Angeles: Billboard Publications Inc. 84 (27): 50. 1 July 1972. ISSN 0006-2510. Retrieved 9 April 2016.
- "Daniel Cassidy: There’s a Sách úr Born Every Minute", Best American Poetry, Terence Winch, July 07, 2009
- MacDougall, Heather (Fall 2007). "Review of How the Irish Invented Slang: The Secret Language of the Crossroads". The Canadian Journal of Irish Studies. 33 (2): 74. doi:10.2307/25515689. JSTOR 25515689.
- "Archived copy". Archived from the original on June 29, 2013. Retrieved 2014-03-24.
- cassidyslangscam: A debunking of Daniel Cassidy's theories about Irish and slang, archived June 29, 2013.
- The Great Daniel Cassidy Slang Scam!, March 18, 2013, archived June 29, 2013.
- John Madziarczyk, Book Review of "How the Irish Invented Slang" Part II—Side by side definition comparisons., August 9, 2007, archived June 29, 2013.
- Grant Barrett, Humdinger of a Bad Irish Scholar Archived June 29, 2013, at WebCite, November 9, 2007, archived June 29, 2013.
- Ed Power, How we gave the Yanks the gift of our gab, Independent.ie, December 4, 2007, archived June 29, 2013.