Daniel Cassidy, in his atrocious book How The Irish Invented Slang, claimed that ‘jack’, a slang term for ‘money’ and the probable origin of ‘jackpot’, comes from the Irish tiach. Cassidy defines tiach as ‘a small purse, a wallet, a budget fig. money’. Now, there are two common words for a wallet in Irish: sparán (equivalent to the sporran of Highland dress) and vallait. Tiach is not a bag used for money, as far as I know. Furthermore, even if it did mean wallet rather than satchel, why would it figuratively mean money? Do people ask if someone has lots of wallet? They certainly don’t ask if they can borrow some sparán in Irish, never mind tiach!
Then there is the issue of pronunciation. Tiach is not pronounced like jack or jah. It is pronounced (roughly) chee-ah, with the ch of English cheese, or tee-ah in the south, so why would it become jack? (Cassidy didn’t understand Irish pronunciation at all.)
And then there is the fact that jack was a term for a coin in English by the 16th century. It is not completely impossible that an Irish term might have come into English this far back, but it is pretty unlikely.
All in all, Cassidy’s claim is as stupid and as worthless as the vast majority of the claims made in this book.