While í is one of the many plural endings, and one of the first that learners learn, with words like cailíní and buachaillí, not all words that end with í are plurals (just as not all words that end in "s" in English are plural - "news", "bus", "class", for example).
These singular words that end in í usually get a plural ending of -ithe - geansaithe, ainmhithe, eolaithe, lúthchleasaithe, sochaithe.
Jersey, jumper and sweater overlap in British English much more than in American English. All three can mean the woollen or artificial fibre garment that Americans would call a sweater. The outlier is jersey, which can mean sweater, but also a sports top for football, soccer or rugby. As I am sure you know, the Jerseys and Guernseys you mentioned are named after (for) the islands of those names in the English Channel. And their milk is delicious.