Thank you. I normally associate the ending "í" with pluralisation and got this wrong because of that. Am I incorrect about the significance of that specific ending, or is there some specific reasoning behind this?
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.
What does "jersey" mean here? Like a football jersey? I don't think of that as being similar to a sweater at all.
In rural Ontario, a Jersey is a breed of cattle, similar to a Guernsey. Both produce absurdly creamy milk.
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.