* It's misspelled: Could you be thinking of a different word? Perhaps a similar-sounding word or a word with a slightly different spelling?
* It's a dialectal word: Irish has many dialects, and words can vary significantly between them. "Paiste" might exist in a particular regional dialect.
* It's a loanword: It could be a word borrowed from another language and used informally in some Irish-speaking communities.
To help me understand what you mean, could you provide more context? For example, where did you see or hear this word? What was the sentence it was used in?