The suffix -ín in Irish is the diminutive, right? Is there a rule for the gender of nouns ending in -ín? Are they all masculine?
Yep. They're all masculine. That's why cailín is masculine, actually!
The nouns ending with -ín that are diminutives are all masculine. Nouns ending with -ín that aren’t diminutives aren’t necessarily masculine, e.g. aintín.
It's hard to believe that the ín in lachín ("duckling") isn't a diminutive, but lachín is a feminine noun.
Does this refer to a (living) human model of clothes, etc?
How about proper names, like Caitlin?
Caitlín is a diminutive (of Caitríona).
This requires me to learn the correct spelling for two words if I choose to use the word mannequin instead of model :-)