There are no rules. There are not even any patterns. Here are some random words off the top of my head that consist of a noun followed by -ie or -y, but do not try to draw any rules from them. None of them, however, uses -y in any kind of "noun-forming" way. I think that in these examples, "-y" and "-ie" are actually more like a diminutive; [noun]+y means "connected with [noun] in some way." 1) A "foodie" is a person who is deeply interested in food, cooking, and fine dining; it is the modern word for "gourmet" or "gastronome," which are now old-fashioned. Here, "-ie" is really a diminutive. In the state of Massachusetts, the state police are called are "staties;" a store for buying liquor, formally a "package store," is a "packy;" people who are born, grew up, and live their adult lives in a single town are "townies." 2) A "rocky" path or road is a road that is rough or bumpy. Come to think of it "bumpy" is a noun+y form, meaning "full of bumps." 3) "Itchy" means "having a lot of itches." "Rocky," "bumpy," and "itchy" resemble adding "-ish" to an adjective to make it intentionally loose or vague; "reddish" means somewhat red."

Unfortunately, you have to learn each word individually. English vocabulary comes from a number of different languages and so there are no overarching principles to the formation of related words. After a while, you can see patterns and this can help you to guess in new cases. In this case, both of these adjectives derive from French.