IPA: /ˌɪrɪˈzɪtəbl/
KK: /ɪrɪˈzɪstəbl/
Something that is so attractive or appealing that you cannot resist it.
The dessert was so delicious that it was simply irresistible.
Irresistible → It is formed from "in-" (meaning not), "resistere" (meaning to resist), and "-ible" (meaning capable of). The word "irresistible" means not capable of being resisted or unable to be opposed.
Think of something that is 'not' ('in-') able to be 'resisted' ('resistere') — that's why we say it is irresistible.